[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
I am not sure to understand what is the take-home message from this example.

In the case of ensembles, it should be made clear that the prediction variance of the individual trees is not the same as the prediction variance of the ensemble itself. As such, you should improve this example with a bit of interpretation about what you are trying to show.