Web Fonts FTW!

* Use Clear Sans for body text, Fira Sans for headings.
* Add the font middleware and font sets

Additional work to be done:
* The font set needs updated to serve more minimized locale specific fonts.
* The grunt build step needs to be updated to include fonts.css for each locale to minimize the number of requests.

fixes #196
Merge pull request #198 from mozilla/issue-196-fonts

Web Fonts FTW!