Add micro- and millsecond period intervals.
I'm totally willing to implement this myself, but I'm not exactly sure where to begin. I tried adding a microsecond period, but it seems like there are many low level changes required (plib.pyx, period.py (obviously), period.c, period.h, datetime.pyx, datetime.pxd and others that I can't remember off the top of my head). 

Is there some documentation or a list of files and functions that need to be changed to add a period interval? If not, I can try and assemble one.
