ENH Sparse multilabel target support

All multilabel metric calculation is now performed efficiently over
sparse CSR matrices.
DOC label indicators are clearer as ints than floats
DOC more precise input type for multilabel metrics
DOC:Typo in doc

Fixes #3553
DOC GP fixes in what's new
Merge branch 'pr/3395'
DOC correct default value
FIX set vectorizer vocabulary outside of init
Deprecate vectorizer fixed_vocabulary attribute
DOC add missing details to what's new
MAINT simplify covertype benchmark
DOC correct shape of Tree.value
ENH multinomial logistic regression using L-BFGS
ENH: Add support for class weights
FIX: Change alpha to 1./c
ENH: Merge MultinomialLR into LR with multi_class='multinomial'
ENH Added multinomial logreg to plot_classification_probability.py
ENH: Added multinomial option to LogisticRegressionCV
COSMIT: Minor doc fixes
TST: Tests for multinomial logistic regression