helper for comparing histograms

I've added a helper class to compare "stuff" and a concrete implementation for comparing histograms, as well as an example. The expected output is attached. I'm aware that it needs some work, but before I go into polishing I'd like to discuss if you find it helpful in the first place.

![compare](https://f.cloud.github.com/assets/1502412/648808/9c41a1a6-d435-11e2-86d3-3ca4e1f0c434.png)