Design unification

This brings the Training Kit site look and feel (roughly) up-to-date with that of the [Training.GitHub.com](https://training.github.com) site.

The stylesheets and selectors need considerable cleanup and simplification, but this brings the design to a minimally viable and usable state.

![screen shot 2014-04-21 at 2 14 15 pm](https://cloud.githubusercontent.com/assets/352082/2758484/d03a17fc-c991-11e3-9639-bd958818f287.png)
![screen shot 2014-04-21 at 1 38 22 pm](https://cloud.githubusercontent.com/assets/352082/2758483/d039e9f8-c991-11e3-8b03-7795413b12a6.png)
![screen shot 2014-04-21 at 1 38 46 pm](https://cloud.githubusercontent.com/assets/352082/2758485/d03a8908-c991-11e3-8f75-d714d0ecb59d.png)