Adds Side Menu Layout to Pure Site

This pull request adds the much-wanted Side Menu layout. It works just like the navigation menu on the Pure Site. I separated out the menu CSS into its own file and added some comments so that it's easy for people to pull in and modify.

I think this new layout is actually slightly better looking than the one on the Pure Website. :smiley: 

Closes [#164](https://github.com/yui/pure-site/issues/164). 

## Obligatory Screenshot

![](http://f.cl.ly/items/351E213x2C3G3J1H280B/side-menu@2x.jpg)