ENH Function auc_score should throw an error when y_true doesn't contain two unique class values.

TST Added a new test case for the auc_score function (non-binary class values).

PEP8 There should be two blank lines between top-level functions.

PEP8 Removed unnecessary backslashes.

TST Extended the test to include a case where y_true contains only zeros, -1s.
changed warnings in grid_search.py related to loss_func and score_func being passed