datetime review part 2

[Jonathan M Davis]

Most recent code: http://is.gd/hO85C

I added code for Mac OS X which should use its monotonic clock functions. 
Unfortunately, as I don't own a Mac, it's completed untested, but in theory, 
what's there should work, and someone with a Mac can fix it later if need be.

I also updated my unit testing functions again (based on the feedback in the 
thread on a potential std.unittests).

- Jonathan M Davis


P.S. Remember that you need to link in advapi32.lib on Windows.