Use github-style code highlighting

Much nicer. :)

![screenshot from 2013-11-12 17 53 39](https://f.cloud.github.com/assets/191754/1522687/87e0c2e4-4bab-11e3-8f0e-7449f7f2396f.png)

Left: stock pygments.css.  Right: github-style pygments.css.