Added decision_function support to OneVsRestClassifier and a test, test_ovr_single_label_decision_function, in test_multiclass.py
Updated fixes for #2012.
Strengthened tests for OneVsRestClassifier decision_function
Cleaned up tests, and removed unused multilabel parameter in decision_function_ovr
Inlined extraneous function call from decision_function and added a check that the base estimator has a decision_function attribute
PEP8