adding classification_table

I needed to see the actual numbers for the true/false positives and negatives for a project I was working on. I noticed sklearn didn't have an implementation of a table like this, so I coded this up. I'm relatively new to coding, and very new to contributing to large projects like this, so the code may not be at an adequate level, but I thought this would be a feature that would be useful to some people. The implementation draws heavily from the code in `metrics.classification_report`.