text: Title: Default base_estimator for Optimizer
  Body: Currently, the constructor of `Optimizer` expects `base_estimator`. 
  
  It would be a good default I think to have the same GaussianProcessRegressor as in `gp_minimize` when `base_estimator=None`. 
  
  (Reproducing that logic is not trivial for the user, hence that would be a nice shortcut.)