Add support for OS X using Mach clocks.

Hi,
This commit adds support for OS X.
Amongst other sources, I used these 2 links as a reference:
  * http://stackoverflow.com/questions/11680461/monotonic-clock-on-osx
  * http://boredzo.org/blog/archives/2006-11-26/how-to-use-mach-clocks

Please let me know if you need something changed/corrected.

Cheers