ENH add multilabel support to PRF metric family

Support also in classification_report
DOC fix typos
COSMIT shorten long line for pep8
ignore PDFs
ENH Create FeatureSelectionMixin for shared [inverse_]transform code

Also rename FeatureSelectionMixin -> SelectorMixin -> _LearntSelectorMixin
And rename sklearn.feature_selection.{selector_mixin -> from_model}
DOC rewrite descriptions of P/R/F averages and define support
DOC/COSMIT fix typos in What's New
DOC add some contributions to What's New
BUG: set random state in LogisticRegression
DOC clarification in Scoring objects: Its not a good sign if I don't understand my own wording.