Lightweight modes upon existing MulticlassClassifier to be able to process "pair-wise classification" performances instead of "full" multiclass decisions

Should make possible to avoid any voting etc done internally thus returning results for each pair-wise classification simply by providing 'combiner=None' to MulticlassClassifier, e.g

mclf = MulticlassClassifier(clf, combiner=None)
mcv = CrossValidation(mclf, NFoldPartitioner(), errorfx=None)

would simply output all the results which could later be post-processed.  I have a draft of PairwiseBinaryFxNode(BinaryFxNode)  which would estimates hits or misses or any other errorfx on each pair (postproc then of mcv could be used to compute means)