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
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
FIX: np.abs might not work on scipy.sparse matrices

On current scipy master np.abs(X) triggers:

    TypeError: __numpy_ufunc__ not implemented for this type.

I don't know if this is really a bug in scipy or not but in the mean
we can workaround this limitation as this is only for a test anyway.
DOC Remove redundant parameter docs
MAINT remove deprecated functionality from SGD
ENH use threads instead of multiprocessing in SGD