Implementing multilabel confusion matrix - Issue #3452

Perhaps rather than the ordering being arbitrary, let's make this a proper contingency matrix, e.g. `[n_classes, 2, 2]` where axis 1 indicates false/true and axis 2 indicates negative/positive. So true positives for each class can be accessed by `C[:, 1, 1]` (or `C[:, True, True]` but it's believable that such indexing will be deprecated in numpy).