Enhances ClassificationReport documentation
Enhances ClassificationReport documentation

I've added a bit more detail in the classification report documentation
about how to interpret each of the metrics and compare the reports
against each other. I've also confirmed that the docstring bug that
listed colormap as the attr for changing color has been fixed and now
correctly lists cmap.

Fixes #299

pdamodaran/yellowbrick,DistrictDataLabs/yellowbrick