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
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