windows.h was only being included for MSVC, while in UnitTestTimer.cxx, the
Windows function Sleep was being called after check for _WIN32. This was
causing compilation failure in MINGW.
Fixes#122
We previously included windows.h in numerous locations using different
techniques to guard against bringing in parts of the file that are bad
(min/max macros, etc). This solves the problem by consistently using
vtkm/internal/Windows.h to setup everything.