Add support for github enterprise as a hosted repository. This now uses the github v3 api for access. Some rewrite of the scmtools/forms.py was needed in order to pass down the repository extra information, such as the github enterprise hostname, for service authentication.

Note: this is similar functionality to what Edmundo submitted (http://reviews.reviewboard.org/r/3376/) This should be a more minimal change and all the repo settings are stored in the model (rather than settings_local.py).

Added unit tests that passed. Configured public and enterprise repositories.