ENH allow y to be a list in GridSearchCV, cross_val_score and train_test_split.
add test for safe_indexing, add another test for cross_val_score
ENH rename parameters in MockListClassifiers.
TST non-regression test for CV on text pipelines
Merge pull request #2694 from amueller/allow_y_lists

[MRG+1] allow y to be a list in GridSearchCV, cross_val_score and train_test_split