adding support for class_weight in fit method
adding warning if passing class_weight through fit
changing warn message, making it Deprecation Warning, and removing negative index for py2.7
adding simplefilter for warning
changing warning test to use assert_raises_message
COSMIT: Make how to use partial_fit render properly
ENH: Use check_X_y in pairwise_distances_argmin
Merge pull request #3962 from MechCoder/check_X_y

[MRG] ENH: Use check_X_y in pairwise_distances_argmin
Merge pull request #3931 from dsullivan7/cwwarn

[MRG+1] raising warning for class_weight in fit method
CLN: Fix typo in comment
Merge pull request #3964 from bwignall/boostdoctypo

CLN: Fix typo in comment
Fixed a bug in RFECV when step != 1
Merge pull request #3824 from nmayorov/rfecv_bugfix

[MRG+1] FIX: Bug in RFECV when step is not 1