[MRG] rewrite precision_recall_fscore_support

* Rewritten for efficiency and neatness. Note the handling of multilabel-sequences format by binarizing is in response to benchmarks. Under the simple benchmarks at https://gist.github.com/jnothman/5734967, this new version is about 1.3x faster than master (for multiclass and label indicators; about 2x faster for sequence of sequences).
* Also fixed some warning handling in tests