Update stylesheet to include proper copyright and license attribution in style.css. Fixes #10 and also reverts the changes that were made in 06463db11dd44908c9ba2d5aef2ea70a97e142f7, since we can now use this copyright notice to find out which themes are based on Underscores.
Update copyright/license notice. The capital S in _S felt really weird. Also, shorten copyright text and re-arrange link to Underscores.me.
_s: Re-introduce `nav-` prefix for previous/next links.

This way we can select both links with `[class*="nav-"]`.
Merge remote-tracking branch 'upstream/master'