Allow users to embed code snippets in the article

This how it looks like. We only use highlight.js in the show mode, in edit mode we just have a black box with a white font. We can change the theme we use later on.

![show-mode](https://cloud.githubusercontent.com/assets/2647/2623877/58541376-bd0e-11e3-8d81-292f8e0ef480.png)
![edit-mode](https://cloud.githubusercontent.com/assets/2647/2623878/58541d62-bd0e-11e3-9909-eab42fc0609c.png)