Filter notifications by reason

![github_notifications](https://cloud.githubusercontent.com/assets/173/2634504/02e0ced4-be7b-11e3-817e-1ebb8ba73489.png)

This required quite a bit of refactoring for how the filters work, and there is still more to do to get the lists right. But this somewhat works.

Fixes #65 


/cc @kansaichris