title: Evaluation always starts from num_boost_round = 1 when early_stopping_rounds is specified
body: <p>While tuning for num_boost_round in XGBoost in python,the evaluation always starts from one booster round  when early_stopping_rounds is specified.</p>
  <p>So, when I try to run the following code,
  <code>xgboost.train(params, dtrain, evals, num_boost_round = 100, early_stopping_rounds = 20)</code></p>
  <p>the evaluation by default starts from num_boost_round = 1 and increases further. Is there any way can I start the evaluation from num_boost_round = 10 or 20 and proceed further till a significant improvement in score is reached?</p>