New icons and restyle of the git-panel structure

As usual my merge-requests are messy :(

Ok talking about this new version...

- Replaced .svg icons with **Octicons** iconic font (from GitHub).
- Added icons to every button and removed labels
- Added tooltips to every button and renamed from BUTTON_* to TOOLTIP_* in the language file.
- Restyled the HTML structure of the **git-panel.html**, now there aren't double buttons in the markup.

![image](https://f.cloud.github.com/assets/5382443/2206242/728002b6-995d-11e3-919d-7ab1057527a1.png)

![image](https://f.cloud.github.com/assets/5382443/2206245/7f456b30-995d-11e3-9d41-9339ab838f84.png)

Hope everything it's ok.

https://github.com/zaggino/brackets-git/issues/47#issuecomment-35481364