Record username of the user that pushed commits to the server

Adds new feature on the server to record username of the user who pushed the commits. The feature can be enabled on a per-repository basis; it is off by default. 

* User that pushed commits to the server is recorded in git notes associated with that commit.
* Commits view has been adjusted to show all notes on the commit
![commitmsg](https://cloud.githubusercontent.com/assets/4617066/3955462/ddbaca52-2701-11e4-8c23-63aa53ecfeb9.png)
* There's a new option on repository edit page named "Audit Pushes"