What is claimed is: 1. Apparatus for synchronizing the repetition rates of two pulse radiation sources comprising: means for controlling the repetition rate and hence the timing of a designated one of the sources with respect to the other source in response to a control signal; a first synchronization stage for roughly synchronizing the two sources, by generating a rough feedback signal comprising the control signal; and a second synchronization stage for finely synchronizing the two sources, by generating a fine feedback signal comprising the control signal. 2. The apparatus of claim 1, further including a repetition rate control device for initially providing the rough feedback signal as the control signal to the designated source, and then for gradually shifting to provide the fine feedback signal as the control signal to the designated source. 3. The apparatus of claim 2 wherein the sources are mode-locked lasers. 4. The apparatus of claim 3, wherein the lasers are Ti:Sapphire lasers, and wherein the jitter between the two lasers is reduced to less than 15 femtoseconds. 5. The apparatus of claim 4, wherein the lasers have a common repetition rate. 6. The apparatus of claim 5 wherein the first synchronization stage synchronizes the fundamental frequency of the common repetition rate and wherein the second synchronization stage synchronizes a harmonic of the common repetition rate. 7. The apparatus of claim 4, wherein the lasers have different repetition rates. 8. The apparatus of claim 7 wherein the first synchronization stage synchronizes lower harmonics of the repetition rates and wherein the second synchronization stage synchronizes higher harmonics of the repetition rates. 9. The apparatus of claim 2, further comprising: a third synchronization stage for extra-finely synchronizing the two sources, by generating an extra-fine feedback signal comprising the control signal. 10. The apparatus of claim 9, wherein the extra-fine feedback signal is based upon a combination of the laser beams within a nonlinear device. 11. The apparatus of claim 9 wherein the extra-fine feedback signal is based upon a heterodyne beat of the laser beams. 12. A method for synchronizing the repetition rates of two pulse radiation sources in stages, comprising the steps of: generating a rough feedback signal from an error signal based upon lower harmonics of the repetition rates of the sources; generating a fine feedback signal from an error signal based upon higher harmonics of the repetition rates of the sources; and generating a control signal based upon the rough feedback signal and the fine feedback signal and controlling the repetition rate and hence the timing of a designated one of the sources with respect to the other source in response to the control signal. 13. The method of claim 12, further including the steps of initially providing the rough feedback signal as the control signal to the designated source, and then gradually shifting to provide the fine feedback signal as the control signal to the designated source. 14. The method of claim 13 wherein the sources have a common repetition rate, wherein the step of generating the rough feedback signal utilizes an error signal based upon the common repetition rate and wherein the the step of generating the fine feedback signal utilizes an error signal based upon a harmonic of the common repetition rate. 15. The method of claim 13, further comprising the step of: generating an extra-fine feedback signal; and wherein the step of generating the control signal generates the control signal further based upon the extra-fine feedback signal. 16. The method of claim 15, wherein the extra-fine feedback signal is based upon a combination of the source beams within a nonlinear device. 17. The method of claim 15, wherein the extra-fine feedback signal is based upon a heterodyne beat of the source beams. 18. Apparatus for synchronizing the repetition rates of two pulse radiation sources in stages, comprising: means for generating a rough feedback signal from an error signal based upon lower harmonics of the repetition rates of the sources; means for generating a fine feedback signal from an error signal based upon higher harmonics of the repetition rates of the sources; means for generating a control signal based upon the rough feedback signal and the fine feedback signal; and means for controlling the repetition rate and hence the timing of a designated one of the sources with respect to the other source in response to the control signal. 19. The apparatus of claim 18, further comprising means for initially providing the rough feedback signal as the control signal to the designated source, and then gradually shifting to provide the fine feedback signal as the control signal to the designated source. 20. The apparatus of claim 18 wherein the sources have a common repetition rate, wherein the means for generating the rough feedback signal utilizes an error signal based upon the common repetition rate and wherein the means for generating the fine feedback signal utilizes an error signal based upon a harmonic of the common repetition rate. 21. The apparatus of claim 18, further comprising: means for generating an extra-fine feedback signal; and wherein the means for generating the control signal generates the control signal further based upon the extra-fine feedback signal. 22. The apparatus of claim 21, wherein the extra-fine feedback signal is based upon a combination of the source beams within a nonlinear device. 23. The apparatus of claim 21, wherein the extra-fine feedback signal is based upon a heterodyne beat of the source beams. 24. The apparatus of claim 18, further comprising means for adding a dc offset signal to the control signal.