New style for history tab.

- Now everything it's in a single place (the tab), no popups anymore.
- On the left you can see the history of commits with users avatars took from [Gravatar](http://gravatar.com/).
- Clicking on a commit, the choosen one is loaded on the right side of the history tab.
- Comments are now formatted using markdown.

![image](https://cloud.githubusercontent.com/assets/5382443/2581029/016e63e8-b9b4-11e3-8950-a1e8112f6925.png)