chore(tests): add coverage as an option

Knowing what exactly isn't being covered by the tests is more important than knowing the statistics.
You can find bugs and dangerous code paths that haven't been taken.

By default, this outputs full HTML reports with indicators of which lines or branches haven't aren't covered.

Here's a screenshot for an untested part of carousel.js:
![screen shot 2013-12-28 at 4 43 05 pm](https://f.cloud.github.com/assets/132584/1818715/70a3d904-7022-11e3-8bba-e054cd117f86.png)

You can now run this with the --coverage option.