[MRG] Prediction variance in bagging-based regressors

This addresses issue  #3271. The main points are
 * Option with_std added to method predict of BaggingRegressor and RandomForestRegressor. If True, the standard deviation of the predictions is returned in addition to the mean
 * Added option with_std to GaussianProcess.predict() and deprecated eval_MSE, which returned the predictive variance
 * Added one example comparing the predictive distributions of the three methods.

Please also check the planned deprecation path of the parameter eval_MSE in GaussianProcess
Please mention that this is optional, like it is done below for random forests.