improve coverage

Fixes #333 
It's not a pretty diff, but it will put us back in the green.

![screen shot 2014-12-16 at 4 46 17 pm](https://cloud.githubusercontent.com/assets/723048/5462555/146e4368-8543-11e4-8f7a-3c5767622209.png)

Added this because it looks awesome: https://github.com/twada/power-assert#see-the-power-assert-output-appears