chore(test): Add test coverage task

Run `grunt test:coverage` to see a the test coverage in the terminal.

Creates output that looks like this:
![screen shot 2013-12-24 at 3 55 49 pm](https://f.cloud.github.com/assets/132584/1807547/ee4a47dc-6cf6-11e3-9dbb-ea257a402894.png)