[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
If you skip RF, just don't mention them in the example description. This is confusing. Or, you could also use bagging with an SVR (using RBF or polynomial kernel). This should produce different results from RF.