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
FIX use fixes.bincount for numpy 1.3.0 support
FIX Py2.6-compatible use of str.format
ENH bagging meta-estimator
ENH: move _partition_estimators to ensemble.base
TST: test_base in sklearn.ensemble
BUG: do not force base estimators to inherit from sklearn in AdaBoost
TST: fix random_state in test_weight_boosting
DOC: base_estimator_ + estimators_ in ensemble.base
COSMIT: tidy up forest code (inspired by bagging)
Merge branch 'bagging'

Fixes #2375.