Add four spaces markdown for raw code blocks

Add `gfm.code` to four spaces code blocks.

Also specify `raw` class so we can apply specific styling like this :

![capture decran 2014-03-01 a 01 28 38](https://f.cloud.github.com/assets/143380/2300821/fa4fb9f2-a10a-11e3-8e26-f7f6c4907d91.png)