Add description and repository url

Fixes package appearance on npmjs.org and http://addons.builtwithember.io

Currently looks like this:
![screen shot 2014-09-22 at 9 20 48 pm](https://cloud.githubusercontent.com/assets/61048/4366484/e6b4cc78-42bf-11e4-9db3-0d849a83b1af.png)