Added options to the Github repository form to enable posting

    to a Github issue when a bug is first mentioned or when a

    review request referencing a bug is updated and closing a

    Github issue when a Review Request is closed.

Added properties to Repository for the new bug tracker 

    integration options.

Added new methods to HostingService that are called when the 

    review request is closed or updated to take care of these

    deeper bug tracking options.

Added the ability to post to bug on first reference.

Added a bug tracker form to HostingService.

Updated RepositoryForm to use the new bug tracker form in the 

    bug tracker section if it exists.

Created unit tests in reviewboard.hostingsvcs.tests.test_github.py

Tested everything with a private GitHub repo and a local reviewboard instance.