Remove use of class names as metrics_hash keys

As a follow on to the previous PR fixing the bug caused by using class names as hash keys, I took a stab at removing the use of class names as the metrics_hash's keys, replacing them with a `type` attribute on the generator. I had originally decided to spend my weekend addressing #20 before fixing bugs, and decided that it might be best to have both the original churn calculator as well as a new one based on the Churn gem, as the Churn gem doesn't support perforce (and I don't know enough about it to work on a PR - sorry, @danmayer!)

One interesting wrinkle to this change is that it would allow for the addition of Calculators of many different metric types. Mashing up metrics might be a useful 2.0 feature..

I'm not 100% on this change, and am actively seeking feedback.