Correlate Status events with Pull Requests

[ulfsauer0815]

You would like to have status messages on pull request events.
Status events do not have a reference to pull request events - they are just related to a particular commit.

In order to make that association, status events need to be correlated to previous pull request events.
Pull request events could be cached for some time to achieve this.

// correlation
pullrequest.after == status.sha

// same payload
pullrequest.after == pullrequest.pull_request.head.sha
status.sha        == status.commit.sha