Add Flat-UI

Per your request, adds Flat-UI from Darthdeus' gem, [flat-ui-rails](https://github.com/darthdeus/flat-ui-rails/blob/master/README.md).

It seems to look good; some green newly shows, etc.

Flat-UI is loaded after Twitter Bootstrap per this [answer](https://github.com/darthdeus/flat-ui-rails/issues/6#issuecomment-15594209) to your question at the gem.

Of course Twitter Bootstrap must be first to avoid clobbering the others with the style normalization.