Feature icons

I have moved all icons to use FontAwesome icons because we can't use the OctIcons from GitHub interface.
I have used these icons:
- [fa-tag](http://fortawesome.github.io/Font-Awesome/icon/tag/) for tags
- [fa-code-fork](http://fortawesome.github.io/Font-Awesome/icon/code-fork/) for heads (branchs)
- Made by me for single commit (circle with a vertical line)
I have also removed the useless xfc (gimp) file in order to keep the repository clean.