Improvements to contributors page

The contributors page wasn't loading the gravatar urls (feed wasn't providing them) and it didn't have any real formatting applied.

## Design Changes
**Current design:**
![image](https://cloud.githubusercontent.com/assets/1038062/5039335/824f9f4e-6b94-11e4-83ab-84d626291489.png)
**Proposed design:**
![image](https://cloud.githubusercontent.com/assets/1038062/5039349/95a75b72-6b94-11e4-932b-5b46c7107d58.png)

I wanted to get the full name of the developer in the `h4` tag but after consulting the documentation the only way I could see to pull it in was to query the api for each contributor. I didn't do that as it seemed like overkill.

## Licensing Considerations
I used a small snippet of Bootstrap 3 markup which is licensed under the MIT License:

  * https://github.com/twbs/bootstrap/blob/master/LICENSE

**Key elements**
  > Permission is hereby granted, free of charge...to use, copy, modify, merge...subject to the following conditions:
  > 
  > The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

I used approximately 50 lines of the css code (and some of that could be trimmed down to be honest). My personal feeling is that this doesn't qualify as a "substantial portion" of the software so I don't think its an issue.