Add ClassBalance class about yellowbrick

[rebeccabilbro]

Suggest using precision_recall_f1_support method from sklearn metrics to get the 4th item in the tuple, which is the support score.

[rebeccabilbro]

Preliminary implementation complete: https://github.com/DistrictDataLabs/yellowbrick/blob/develop/yellowbrick/classifier.py#L179.

Coloring method will need to be refactored.

[bbengfort]

I'd like to use the yellowbrick.style.palettes module as well! I suggest we move that to a new issue, and make sure we keep this class up to date per #44 -- but other than that, if this implementation works, then let's close this issue! Also - you have issues for the tests; so I think you've achieved this card!

[rebeccabilbro]

Marking as complete, with the understanding that issues #62 will resolve the fig/ax hook question that came up in the design of the ScoreVisualizer class and that #67 will resolve the initial tests requirement.