Keep track of PR commit hashes

[timothyb89]

It'd be nice to use the pr bot to send notification on CI/CD pass / fail for pull requests. Unfortunately status events don't include any details about their associated pull request and only have the commit sha. To send notifications properly, we'll need to keep tracked PRs and their hashes in the database.

Realistically this means fleshing out https://github.com/monasca/pr-bot/blob/master/lib/pullrequest.js more fully.