add support for https:// based repositories at github

Currently reviewboard's github hosting service supports only git:// repositories.
It would be nice to support https:// based repositories as well. This patch provides such functionality
by adding a drop-down list for github access protocols.

I am able to create a repository with https:// and post a review request with post-review tool. Deployed on reviewboard instance of dCache.ORG project ( http://rb.dcache.org ).