Add Github buttons

Fix https://github.com/hoaproject/W3/issues/20.
![capture d ecran 2014-11-07 a 11 39 22](https://cloud.githubusercontent.com/assets/946104/4951701/98099396-666a-11e4-8f74-85489d9569fa.png)
Thoughts?

/cc @hoaproject/hoackers