Added the hover effect on documentation code to improve legibility

I added the :hover css effect to expand the code box in order to improve legibility.

This is the actual situation:
![screen1](https://cloud.githubusercontent.com/assets/475967/3038612/f5aa5740-e0cf-11e3-8523-4ab027d2f29e.png)

With this PR the box will be expanded as follow, only on mouse hover:
![screen2](https://cloud.githubusercontent.com/assets/475967/3038619/06b9d31c-e0d0-11e3-8f94-80fd00c53301.png)