Add MCC to evaluation measures for classification

[janvanrijn]

As requested in openml/OpenML#191

I received a request to add the Matthews correlation coefficient to the evaluation engine for classification. It can be straightforwardly derived from the confusion table: MCC = (TP * TN – FP * FN)/sqrt((TP+FP) (TP+FN) (FP+TN) (TN+FN)).

I propose we check if it is available in Weka, in that case it should be easy to add.