Integrate GitHub pull requests as Differential reviews

Summary:
This uses Doorkeeper to import GitHub pull requests as Differential revisions.  For this to work, users need to do two things:

  * Create a GitHub service hook to point to https://my-phabricator-install.com/doorkeeper/github-hook/ for the repository they want to monitor, with the payload version "application/vnd.github.v3+form".  This can be done by going to "Settings" on the GitHub repository and selecting "Webhooks & Services".
  * Create a bot / service account with the username `github`.  Currently this is hardcoded, but it should really be a configurable option.  This is required because anyone can submit a PR on GitHub, including users which haven't connected Phabricator <-> GitHub accounts.  I don't know whether we want to auto-create accounts in this case instead of using a service account?

Test Plan:
Follow the instructions above, or submit a pull request to https://github.com/hach-que/test and then check https://code.redpointsoftware.com.au/differential/ to see if it appeared.

Also, an example:

**Pull Request:** https://github.com/hach-que/test/pull/2

**Differential Revision:** https://code.redpointsoftware.com.au/D269

Reviewers: #blessed_reviewers, epriestley

Subscribers: spicyj, Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D8775