'Return the label probability distribution for classifying a string
of text.
Example:
>>> classifier = NaiveBayesClassifier(train_data)
>>> prob_dist = classifier.prob_classify("I feel happy this morning.")
>>> prob_dist.max()
\'positive\'
>>> prob_dist.prob("positive")
0.7
:rtype: nltk.probability.DictionaryProbDist'

def prob_classify(self, text):

    text_features = self.extract_features(text)
    return self.classifier.prob_classify(text_features)
