Kill off benchmark comparison as a feature

I never really reviewed this, never use it, and find the code very
obtuse and difficult to understand. This makes it a huge pain to
refactor when I want to add new features.

It seems the more-correct thing to do is to get rid of this feature
and find a better way to implement it that doesn't make the core
of criterion less maintainable.
Filter out zero-valued clock differences

--HG--
extra : amend_source : 63f2ab63ed918284285bfafe2e97108ad7203711
Use a more suitable wallclock time source on each major platform

We now use clock_gettime on most Unix variants, mach_absolute_time
on OS X, and QueryPerformanceCounter on Windows.

Each of these counters provides much better resolution than their
predecessor (getPOSIXTime), and generally at lower cost, too. Nice!