Make it possible to pickle a fit `GridSearchCV` and `RandomizedSearchCV`

Fix for #1789:
* don't make `best_estimator_`'s methods available by copying its methods. Instead link to its attributes by properties (also, added properties for `decision_function` and `transform` where `predict`, `predict_proba` and `score` had been available).
* and don't declare `CVScoresTuple` in a closure.