Added GitHub model and hooked it in to Repo

TODO:
Critical UI hooks:

* still need to hook it into the UI
and force people to create a new GitHub before they can create
a Repo.
* Give the Repo form a pull-down for pre-made GitHub installs
it could be connected to.
* Remove form elements for host and api_endpoint
Hooking GitHub object into Home UI
OAuth Integration completed
updated test fixtures
corrected contributer urls

* added GitHub#convert_api_url_to_web
	which converts api urls to web urls
	(useful because user urls don't come in web variants)