fence that code

@seldo I know you love HTML, but there are cases where markdown is the Right Choice. Notice the unhighlighted code blocks here:

![screen shot 2014-12-01 at 5 18 14 pm](https://cloud.githubusercontent.com/assets/2289/5256496/65465da0-797e-11e4-95e0-01aff2eab713.png)

This PR fixes the code snippets to use GFM.