FIX predict_proba status on SGD and SVC when disabled

Using a property trick suggested by @mblondel. The following now
works:

    >>> clf = SVC(probability=False)
    >>> hasattr(clf, "predict_proba")
    False

This simplifies the smoke tests.

Added a note to SVC stating that this should be done in new code.
ENH use hasattr "predict_proba" in bagging

More robust than catching exceptions.
Merge pull request #2910 from larsmans/hasattr-predict-proba

make hasattr(clf, "predict_proba") work with probabilities disabled