Facebook button

Hi Guys,

So this is just adding a new facebook icon to the frontpage. Pivotal Tracker Id:80970490
Hopefully have done the whole pull request correctly.
Let me know if you want anything changed or if I did anything wrong!

Cheers,
Justin.
"Facebook" should be capitalised, and I think we should probably say "Like us on Facebook" as that is the more common call to action.