WIP GridSearch with advanced score functions.

Follow up on #1014.
This PR does the handling of score functions in the regressor and classifier mixins with minimal intrusion into the code.
This enables estimator to overwrite the score function and do whatever they see fit - though that shouldn't be necessary atm.

To complete this PR, I will probably also need to do some minor adjustments to the other CV classes,

I'm a bit tempted to add a test to the "common tests" to see if ``cross_valdation_score`` with AUC works.