Fixes monospace font

The monospace font used for the code snippets did not work on my system, *Ubuntu 12.04 LTS*, and so I have used a CSS [font stack](http://css-tricks.com/snippets/css/font-stacks/) which has a lot of fall backs! See the difference:

*Before*
![screenshot from 2013-08-27 18 53 08](https://f.cloud.github.com/assets/1687642/1036804/9fd217c6-0f43-11e3-9368-ce51ac139866.png)

*After*
![screenshot from 2013-08-27 18 53 23](https://f.cloud.github.com/assets/1687642/1036806/a9b879ba-0f43-11e3-9a26-771751450bed.png)