Tweak markup.

* Add a few ARIA related stuff
* Pass `?theme=xx` when clicking on `navbar-brand`
* Remove redundant `col-xx-x`
* Add missing `container-fluid` in `navbar`
* Fix `toggleCode` to work with Font Awesome 4.x (fixes #394)
* Lots of other markup tweaks

/CC @jmervine @jdorfman