Center wrapper in Medium theme

Fixes https://github.com/chrishunt/marked/issues/6
cc @fncll

The Medium theme is not centered and I never noticed because I had a fixed width set it my Marked settings:

![limit](https://f.cloud.github.com/assets/65323/2477887/0cec7a66-b077-11e3-97fb-12ad65f5c268.png)