New installer.

- Converted to Promises.
- Providing support for Windows.
- Normalize paths correctly based on platform.
Moved the test installer to main install.
Added async to devDependencies so tests can run.
Remove python from build command.
Revert changing the libgit2 library path.
Updated the license date and binding to latest.

I took the latest binding and made it work with the existing to resolve
the build failure.
Unrelated commit: Updated the git_profanity_check app.
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.
Add an appveyor configuration.
Merge pull request #140 from nodegit/new-installer

WIP New installer.