* Refactor benchmark.cc into benchmark_register.cc and benchmark_run.cc

The benchmark.cc file is getting really big and it contains a bunch of
unrelated components. This patch separates the files into two separate
parts. The "runtime" parts and the "registration" parts.

This patch also removes the PIMPL used by Benchmark. Previously we couldn't
have STL types in the interface but now we can. Therefore there is no reason
to keep BenchmarkImp.

* add missing include

* rework windows timers again

* Guard timespec on older Windows versions

* Remove old thread safety annotation workarounds