Manage GitHub repository hubbub hooks with Curry.

When an admin adds a repository, use GitHub's hooks API to receive
notifications related to that repo's Pull Request activity.

Likewise, when an admin removes a repository, unsubscribe from those
notifications.

Implementation details:

* Use the Octokit gem to interface with GitHub's API.
* Create Curry Factory, it uses a RequestBin URL for the callback_url
* Use VCR for recording and playing back HTTP requests in tests
* The Curry::RepositorySubscriber model handles the subscribe and
  unsubscribe functionality. To create a Curry::RepositorySubscriber, it
  must be initialized with a Curry::Repository.
* The callback_url is saved with the Curry::Repository record. It is
  required for both subscribing and unsubscribing to the GitHub event
  hooks.