Added incomplete nvcc support.

Also moved all toolchain scripts into common pynja/scripts directory.
Refactored gcc & msvc environment functions.

Now the environment setup functions for gcc and msvc
are in common files used by the *invoke scripts.

This is in preparation for nvcc support.
Added functional nvcc support.

This allows using nvcc as a full C++ toolchain.
THere is not yet support for nvcc -dlink.
Added nvcc + msvc11 support.

msvc11 is being treated as "vs 2010" for now.