Cleanup contributors list CSS

This removes inline styles, instead using classes and defining the styles
in the css file. It also centers the text alongside the gravatar, and uses
a slightly different way of positioning it

Before:
![Before](https://f.cloud.github.com/assets/23787/389752/36892f9a-a71d-11e2-8c70-bafb85bad9fa.png)

After:
![After](https://f.cloud.github.com/assets/23787/389753/3708c4c6-a71d-11e2-9610-e4ee1460800b.png)