Enable code highlight

This should change code blocks color highlights from this 
![no_highlight](https://f.cloud.github.com/assets/3141711/1459057/5f397800-43bc-11e3-964e-9f00eb5f960c.png)

to this
![highlight](https://f.cloud.github.com/assets/3141711/1459058/76c6a876-43bc-11e3-9191-38641719007b.png)