Add Github Enterprise support

This will allow you to use a GHE install for Oauth and hosting of `REPOSITORY`.

It depends on the master branch of `omniauth-github` because it has [this commit](https://github.com/intridea/omniauth-github/commit/81b97006c61b3ea794017bf98dceeff64d18d945), but since it's currently not in a release I didn't update the Gemfile.

Open to any and all criticism/suggestions for improvement.