Now compiles when AVX/AVX2/AVX512 enabled

Also:
- better MSVC++ support (it *almost* compiles with the Nov 2013 CTP;
  support for constexpr arrays is missing)
- now uses vector operations for dot products even when _mm_hadd_ps is not
  available
- setup.py now monkey-patches the mingw32 compiler class to not link with
  the msvcrt library (which would prevent the extension modules from working)
- removed odd wording in the documented example
- refactored intrinsics.json and added support for intrinsics with multiple
  prerequisites