Regression support for random forest

[ankane]

Hey @asafschers, thanks for this awesome gem 👍

This PR adds support for regression to random forest. Regression does a weighted average of the decision counts. I wasn't sure what to call the key in the return value, so went with response.

[coveralls]

Coverage decreased (-0.4%) to 98.965% when pulling e5d21fc on ankane:master into bdaa7c3 on asafschers:master.

[asafschers]

Looks great, thanks!

[ankane]

Thanks for the fast response.