mac thread_clock implementation

The chrono library seems to be missing thread_clock support for mac (and the posix implementation doesn't work for it).  This is an implementation to add support using the mach / thread_info api.  It should work on Mac 10.4 / iOS 2.0 and higher.  

I was pretty surprised this didn't already exist, but haven't been able to find anything in the mailing list or trac regarding it.  Curious to hear if there was a reason to avoid this implementation or if it's just something no one got around to.