Add GitHub Ribbon and "Join Us"

Close #448.
Fix btn-nav position due GitHub Ribbon
Join Us -> Join
* Shorter - save real estate at the top of the pate
* #448, #462
Change to a css github ribbon
* responsive for smaller screens
* move menu over more - click will always open menu
* #462
Cross browser - stop text wrap in ff
* #462
Merge pull request #462 from r-gaia-cs/github

Add GitHub Ribbon and "Join Us"