Initial support for releases

This adds support for release notifiations. Currently the releases API doesn't return `body_html`, but there is a fix pending. I'll wait to merge this until that is fixed.

![github_notifications](https://f.cloud.github.com/assets/173/2414760/8b510028-aaed-11e3-9f6d-619221b54cff.png)

Fixes #67