title: predict_proba for a cross-validated model
body: <p>I would like to predict the probability from Logistic Regression model with cross-validation. I know you can get the cross-validation scores, but is it possible to return the values from predict_proba instead of the scores?</p>
  
  <pre><code># imports
  from sklearn.linear_model import LogisticRegression
  from sklearn.cross_validation import (StratifiedKFold, cross_val_score,
                                        train_test_split)
  from sklearn import datasets
  
  # setup data
  iris = datasets.load_iris()
  X = iris.data
  y = iris.target
  
  # setup model
  cv = StratifiedKFold(y, 10)
  logreg = LogisticRegression()
  
  # cross-validation scores
  scores = cross_val_score(logreg, X, y, cv=cv)
  
  # predict probabilities
  Xtrain, Xtest, ytrain, ytest = train_test_split(X, y)
  logreg.fit(Xtrain, ytrain)
  proba = logreg.predict_proba(Xtest)
  </code></pre>