Add fetch PR commits and display them in the activity stream

This will fetch the commits for a PR and display them with the activity stream - as seen on GitHub.

This also tries to make our display more "GitHub like" and uses an octicon from #478 

Example:
![debug-3890](https://cloud.githubusercontent.com/assets/33978/4176653/a7a8e162-360e-11e4-8246-3f951a2f1ec3.png)