Add commit changes to notifier

After a successful deployment, it's useful to know *what* was just deployed.

This PR adds a comparison report to the notifier flow. Upon a successful deployment, a comparison report is sent to the notifier.

The deployed revision will always be compared with the revision last deployed to production.

When using the Slack notifier, links are included for the comparison as well as for commits, authors, issues, and pull requests.

![screenshot 2014-10-26 18 08 59](https://cloud.githubusercontent.com/assets/123595/4784765/17adb1ae-5d65-11e4-9a8d-a2a94c624c1b.png)

Yeah, you would. I wonder if we should track that on the deployments model instead of adding another one-off redis key.