ENH: Speed up sparse_coordinate descent
Fix silhouette score n_labels

Fix such that silhouette score raises error
when n_labels is incorrect
Added number of labels for debug and regexp in test
Replaced cython calls for dot operations with BLAS calls
Speed up using typed memory views
Merge pull request #2951 from Manoj-Kumar-S/speed_sparse

[MRG] Attempt to speed up cd_fast.
staged_predict predicts classes, not probabilites
Merge pull request #2964 from abhishekkrthakur/patch-1

staged_predict predicts classes, not probabilites
Added _LearntSelectorMixin in BaseGradientBoosting
TEST: fix test_forest:test_boston (#2965)
typo
Merge pull request #2971 from jyu-rmn/master

typo
DOC: the search for a split does not until one valid partition is found
add datarobot testimonial

use png -- thanks @larsmans
Merge pull request #2973 from pprett/datarobot-testimonial

add datarobot testimonial
DOC/MAINT: clarify cblas/README.txt
Added extra check to feature importances
Merge pull request #2966 from maheshakya/feature_selector

[MRG] Added _LearntSelectorMixin in BaseGradientBoosting
DOC clarify Imputer constructor: arbitrary strings not accepted
Merge pull request #2959 from Oscarlsson/silhouette_score_label_number

[MRG+1] Silhouette score crashed if wrong number of labels