Add a nav link to support site

Also dropped the about us link, since it was only linking back to the
front page, and more links means more awkward wrapping to a second line
on mobile.

In the future we should responsify the nav to turn into a menu button on
mobile.

Oh, and fuck facebook.
Add a support section to the bottom of /

Could be a little wordier, but less is more, right?
Merge pull request #26 from WhisperSystems/support

Add links to desk.com-powered support site