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 basically good; some newly green shows, etc. However, the top of the words, "Your Community" are cut off. Plus, the settings icon is too low. I speculate these will take some styling work; what, I don't know.  Here's how it looks:
![shot](https://f.cloud.github.com/assets/692702/317717/44e415f8-987e-11e2-922e-a61d964e8e3e.png)
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.