Add syntax highlighting of code blocks

Using [highlight.js](http://highlightjs.org/). If you know something better, lemme know.

Some "live" examples:
![highlight-1](https://f.cloud.github.com/assets/213/1458760/d1359fc0-4393-11e3-8f7e-46b1b61423a8.png)
![highlight-2](https://f.cloud.github.com/assets/213/1458761/f9695afe-4393-11e3-85f2-c0ecfe889cb7.png)