Add predict_proba() or decision_function() to each predictor

[mgalloy]

Each predictor should have either a predict_proba() or decision_function() method.

The predict_proba method of a classifier outputs the probability of the instance being in each of the classes.

The decision_function method finds the distance to the separating hyperplane. For example, an SVM classifier finds hyperplanes separating the space into areas associated with classification outcomes. This function, given a point, finds the distance to the separators.