Add the default monospace font stack for <code> elements

Unfortunately `<code>` elements weren't getting the monospace font stack they deserved so they looked a little out of place. :lipstick:

Turns
![github-old](https://f.cloud.github.com/assets/283234/2135391/7b549c1e-92fd-11e3-95a5-0f535c91cb08.png)
into
![github-new](https://f.cloud.github.com/assets/283234/2135392/823f2094-92fd-11e3-9429-2666166b04f0.png)