Supermarket monitors Pull Requests for CLA signage

This is the first part of Curry - a part of the application that checks
the committers of pull requests that are opened insubscribed repos.

Pivotal Tracker Story: https://www.pivotaltracker.com/story/show/64753356

This implements the following:

* Interface for managing the repositories that Supermarket subscribes
  to, which also interfaces with GitHub to create a subscription hook
* Creates an end-point that GitHub sends a POST request to when a pull
  request is updated (opened, synchronize, closed, reopened)
* Checks all of the committers in a pull request, and stores committers
  who have not signed a CLA, identifying those committers either by
  their GitHub login, or if they don't have a GitHub login, the email
  address used to author the commit.
* If all of the committers in a pull request have signed the Supermarket
  CLA, a label is added to the Pull Request saying so.
* If there are users who have not signed a CLA, a comment is left
  instructing those users to do so.

Implementation details:

* Use the Octokit gem to interface with GitHub's API.
* Use VCR for recording and playing back HTTP requests in tests
* Use currybowl for Curry hub callbacks. Only used in Development.
* The checking of PR committers and interacting with the Pull Request
  are handled with Sidekiq background jobs.