Modernize layout

I suggest we use sphinx-bootstrap-theme to give our "Introduction to Programming with Python" a newer, fancier look.  It's going to look like this:
![pybeginners-bootstrap](https://cloud.githubusercontent.com/assets/33013/2654179/308760a6-bfce-11e3-91c4-46df16981a9c.png)

The Sphinx theme also supports using [Bootswatch](http://bootswatch.com/).