Adds description to package.json

Fixes package appearance on npmjs.org and http://addons.builtwithember.io

Currently looks like this:
![screen shot 2014-09-22 at 9 27 14 pm](https://cloud.githubusercontent.com/assets/61048/4366527/c45fd1b2-42c0-11e4-9641-5327094ceea0.png)