Update poor man's profiler to use C++11 std::chrono

Update poor man's profiler to use C++11 std::chrono

This is more portable, and the old code prevented in windows to use
instrument several classes at the same time (several .obj files,
actually).

No new feature here, bu the code is nicely slower. It might be less
efficient, I am not really at ease with how std::chrono works.