title: Xgboost. obtain the booster from the best iteration
body: <p>I am using Xgboost.train to train my model; however, i am not sure how to obtain the booster from the best iteration instead of the booster from the last iteration.  </p>
  
  <pre><code>    xgb1=xgb.train(self.params,xgtrain,num_boost_round=self.num_boost_round,early_stopping_rounds=self.early_stopping_rounds,evals=watchlist)
      print xgb1.best_score
      print xgb1.best_iteration
      print xgb1.best_ntree_limit
  </code></pre>