Use MSVC to compile instead of mingw

This PR adds the minimal changes necessary to build astropy with MSVC instead of mingw.

I made ``run_with_env.cmd`` a hidden file in the main astropy directory rather than putting it and the ``.ps1`` script into a subdirectory...but don't really care too much where it ends up.

Also, note that the 64-bit tests will still fail for now because there really do seem to be three tests that genuinely fail.