Add monospace system of fallback

Before:

![screenshot from 2013-05-10 11 30 06](https://f.cloud.github.com/assets/153842/487328/aa6e15b2-b954-11e2-967d-b28447913653.png)

After:

![screenshot from 2013-05-10 11 29 54](https://f.cloud.github.com/assets/153842/487329/b4ea05c8-b954-11e2-8348-2513d59fb058.png)

See http://www.w3.org/Style/Examples/007/fonts.en.html#font-family