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.

Hope everything it's ok.