Restrict access using GitHub authentication

This will only allow access to users in a certain GitHub organisation. The GitHub application and organisation to use are configurable via environment variables.

The `warden-github-rails` gem stores the authenticated GitHub user's details in the session. Rails 4.1 changes the default session store to be JSON-based, which isn't capable of storing complex objects. I've switched back to the old Marshal-based session store to make it work.