Add description and fixes repository url

Fixes package appearance on npmjs.org and http://addons.builtwithember.io

Currently looks like this:
![screen shot 2014-09-22 at 9 31 16 pm](https://cloud.githubusercontent.com/assets/61048/4366546/58ff73ae-42c1-11e4-85a1-026deeffe9fb.png)