Add "Light center" style

After making a design of Paste-py for Isomorphi.me (alsa used for Prologin's paste), I figured it could be nice to embed it in the official repot.
![capture du 2014-06-05 23 31 41](https://cloud.githubusercontent.com/assets/2143796/3194021/cbf9584e-ecf8-11e3-9ff2-a6074fb8a6b0.png)