Finally, Windows support!

This has taken me hours to figure out, but in order to get Windows
working a few issues had to be resolved:

- Casting. I've added an additional argument parameter in the libgit2
  JSON API to allow for special cast provisioning in the case where
  direct casting isn't possible.

- Update libgit2 build flags to not use STDCALL when compiling in
  Windows.

- Updated the binding.gyp file to have the correct configuratoin
  necessary for cross platform compiling.