COSMIT rewrite precision_recall_fscore_support

* Rewritten for efficiency and neatness. Note the handling of multilabel-sequences format by binarizing is in response to benchmarks.
* Also fixed some warning handling in tests
FIX warning interaction problem, DOC clarify parameters
FIX ignore corrcoef warning
Use label_binarize instead of LabelBinarizer
Use specialised warning class
TST messages for UndefinedMetricWarning from prf
TST use ignore_warnings helper
TST classification_report example that does not trigger warning
Merge pull request #2278 from jnothman/prf_rewrite3

[MRG] faster, flatter precision_recall_fscore_support