Dark mode and Base 16 color schemes added

Added an option for a dark mode for the page, using Bootswatch's Slate theme.

![screen shot 2014-06-15 at 4 12 06 pm](https://cloud.githubusercontent.com/assets/1315101/3280699/71969b42-f477-11e3-9d67-2ffa8ddbba14.png)