code: def importance(self, attribute, examples):
          
          gain_counter = OnlineInformationGain(attribute, self.target)
          for example in examples:
              gain_counter.add(example)
          return gain_counter.get_gain()
docs: AIMA implies that importance should be information gain.
          Since AIMA only defines it for binary features this implementation
          was based on the wikipedia article:
          http://en.wikipedia.org/wiki/Information_gain_in_decision_trees