code: def normalized(self):
          
          total = self.total()
          result = Histogram()
          for value, count in iteritems(self):
              try:
                  result[value] = count / float(total)
              except UnorderableElements as e:
                  result = Histogram.from_dict(dict(result), key=hash)
                  result[value] = count / float(total)
          return result
docs: Return a normalized version of the histogram where the values sum
          to one.