Generate thumbnails for documents.

Adds functionality to generate thumbnails for documents. 

**Dependencies:**
Ubuntu:
* Imagemagick (~50mb). (```apt-get install imagemagick```)
* Wand python library.

Mac:
* ```brew install imagemagick```
* For PDF support, install the Ghostscript dmg (https://www.macupdate.com/app/mac/9980/gpl-ghostscript)
* Wand python library.

![screen shot 2013-11-21 at 11 03 05 am](https://f.cloud.github.com/assets/1141646/1593236/7f755588-52c8-11e3-957a-2e36c506405b.png)