Adds description to package.json

This fixes package appearance on npmjs.com and http://addons.builtwithember.io

Currently looks like this:
![screen shot 2014-09-22 at 9 16 10 pm](https://cloud.githubusercontent.com/assets/61048/4366452/6542e936-42bf-11e4-8986-b6745574cf3a.png)