[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.
Hmm... currently the `binarize` option isn't used (or tested, indeed), but you're right that it would better produce sparse output. I can just remove it. Or as @arjoly suggests, get rid of `_SequencesMultilabelHelper` and use it always. Perhaps that deserves a benchmark.