[MRG] scorer: add sample_weight support (+test)

 Wraps up #3098 (a part of #1574), ready for review.

Initial description by @ndawe:
> This is part of the larger #1574 and adds support for sample weights in the scorer interface.
@amueller The difficulty is that, unlike when testing the metrics directly, for testing the scorer it's tricky to anticipate the output of the classifier / regressor.