Improve Monospace Font Stack on GNU/Linux Systems

DejaVu Sans Mono isn't very easy to read (and  quite ugly) when rendered in the browser on GNU/Linux systems. I propose adding Liberation Mono between Menlo and DejaVu Sans Mono which I think is a big improvement for users who are running similar systems and have this font installed.

> ![screenshot from 2014-11-16 13 00 50](https://cloud.githubusercontent.com/assets/3788970/5061500/51c4675c-6d91-11e4-8730-391568e0b333.png)

> ![screenshot from 2014-11-16 13 00 35](https://cloud.githubusercontent.com/assets/3788970/5061499/518d5442-6d91-11e4-9b39-0a920de0c58b.png)