[MRG] Sparse multilabel target support in metrics

This introduces a series of helper classes that abstract away aggregation over multilabel structures. This enables efficient calculation in sparse and dense binary indicator matrices, while maintaining support for the deprecated sequences format.
All multilabel metrics will support sparse input. There is no reason not to.

What I meant above is that I can put this into `_check_clf_target`, but `_check_clf_target` will need to take a new `labels` (or `label_order`) parameter for binarization