Accept pull req events / pass JANKY_COMMIT_URL

- JANKY_COMMIT_URL is now pushed to Jenkins jobs as parameter
- Hook registration establishes pull_request event
- Payload parser now speaks pull request event
- Add API support for repos/#{nwo}/pulls/#{number}/commits
- Be resilient to GitHub messing with the API results -
originally it was commit["commit"]["message"], then commit["message"],
and then commit["commit"]["message"] once again
Added JANKY_GITHUB_EVENT_TYPES configuration

 - Allows for configuring whether Janky responds to pull_request or
 push event types or both
 - Added parsing of the user provided values to ensure only the
 appropriate ones are allowed
 - Corresponding docs