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.
DOC Fix references to missing examples
fix incorrect reference
Merge pull request #1986 from jaquesgrobler/DOC_reference_fixes

Doc reference fixes
ENH add latent semantic analysis/sparse truncated SVD

Fixes #1519.