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 24 35 pm](https://cloud.githubusercontent.com/assets/61048/4366513/661d56ce-42c0-11e4-989f-4d6a99a8c508.png)