code: def normalize(self):
  		
  		total = sum(self.values())
  		stats = {k: (v / float(total)) for k, v in self.items()}
  		return StatsCounter(stats)
docs: Sum the values in a Counter, then create a new Counter 
  		where each new value (while keeping the original key) 
  		is equal to the original value divided by sum of all the
  		original values (this is sometimes referred to as the 
  		normalization constant). 
  		https://en.wikipedia.org/wiki/Normalization_(statistics)