Admins can add repos for Curry to watch.

Curry allows Chef Admins to keep track of GitHub repositories for which
Supermarket tracks CLA status.

Note: this currently does not subscribe to the GitHub repo's hubbub hub.
This is just to keep track of repos within Supermarket.
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.