title: Why there is no booster parameter in the XGBoost installation, meanwhile it is in the documentation?
body: <p>I have installed the XGBoost.
  These are the parameters it displays when doing:
      print(xgboost.XGBClassifier())</p>
  
  <pre><code>XGBClassifier(base_score=0.5, colsample_bylevel=1, colsample_bytree=1,
     gamma=0, learning_rate=0.1, max_delta_step=0, max_depth=3,
     min_child_weight=1, missing=None, n_estimators=100, nthread=-1,
     objective='binary:logistic', reg_alpha=0, reg_lambda=1,
     scale_pos_weight=1, seed=0, silent=True, subsample=1)
  </code></pre>
  
  <p>But in the documentation <a href="http://xgboost.readthedocs.io/en/latest/python/python_api.html#module-xgboost.sklearn" rel="nofollow noreferrer">Scikit-Learn API</a>, a "booster" parameter" appears. Why I do not have it?</p>
  
  <pre><code>Scikit-Learn API
  Scikit-Learn Wrapper interface for XGBoost.
  
  class xgboost.XGBRegressor(max_depth=3, learning_rate=0.1,  
  n_estimators=100, silent=True, objective='reg:linear', 
  **booster='gbtree'**, nthread=-1, gamma=0, min_child_weight=1, 
  max_delta_step=0, subsample=1, colsample_bytree=1, colsample_bylevel=1, 
  reg_alpha=0, reg_lambda=1, scale_pos_weight=1, base_score=0.5, seed=0, 
  missing=None)
  </code></pre>
  
  <p>EDIT</p>
  
  <p>This is what I get:</p>
  
  <pre><code>    ---------------------------------------------------------------------------
  TypeError                                 Traceback (most recent call last)
  &lt;ipython-input-63-26499cfcb8f8&gt; in &lt;module&gt;()
       18         #Inicio de Cross-validation
       19         clf = Pipeline([('rcl', RobustScaler()),
  ---&gt; 20                         ('clf',    xgboost.XGBClassifier(booster='gbtree', objective='multi:softmax', seed=0,  nthread=-1))])
       21         ##############4 epoch x sujeto###########
       22         print("4 epoch x sujeto en test_size")
  
   TypeError: __init__() got an unexpected keyword argument 'booster'
  </code></pre>