new_contents: class Evaluator(object):
      def __init__(self):
          pass
  
      def fbeta(self, precision, recall, beta=1):
          """
          F1: for bata = 0.5, also called f1 score
          F2: for bata = 1
          """
          return (1+beta**2)*precision*recall/((beta**2)*precision+recall)
  
      def praf(self, tp, fp, tn, fn):
          precision = tp*1.0/(tp+fp)
          recall = tp*1.0/(tp+fn)
          accuracy = (tp+tn)*1.0/(tp+tn+fp+fn)
          fmeasure = self.fbeta(precision, recall, .5)
          print self.__praf_str(precision, recall, accuracy, fmeasure)
  
      def __praf_str(self, p, r, a, f):
          aa = []
          aa.append("Precision: {:.3f}".format(p))
          aa.append("Recall: {:.3f}".format(r))
          aa.append("Accuracy: {:.3f}".format(a))
          aa.append("Fmeasure: {:.3f}".format(f))
          ast = ""
          for a_ in aa:
              ast += "{}\n".format(a_)
          return ast
  
subject: Add evaluator base for #34