Adds support for setting statuses on pull requests.
When a new pull request is created, and that repository is linked to a RB webhook, a new corresponding review request will be created. That will automatically set the pending status for the pull request.
Whenever an issue is open on the review request, the status is set to "error".

When the review request is "approved" (defaulting to 1+ ship it's and no open issues), the status is set to "success".

Otherwise, the PR has a "pending" status.

Added unit tests

to verify the pull request hook is called, and only works with pull_request events and the right signature
to verify pull requests are created and updated when a pull request is created
to verify the pull request's status starts off with 'pending', is 'error' when there's open issues, and 'success' when approved





Manual testing

Created a pull request on a repository linked to ReviewBoard through webhooks
Verified the review request was created with 'pending' status
Verified that opening an issue on the review request sets the 'error' status on the pull request
Verified that having no open issue and adding a 'Ship it!' sets the 'success' status on the pull request