Attribution mark & copyright page redesign

See http://lists.openstreetmap.org/pipermail/talk/2013-April/066802.html

This is a reboot of the previous contributor mark pull request, #180.

Open issues:

 - [ ] Don't mandate, but recommend/offer attribution mark
 - [ ] Make it clear that /copyright is scrollable
 - [ ] Let's do an alt text to make sure we're on Google's radar ;)
 - [ ] Make sure mark is easy to embed