[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.
I would rather have a more explicit test. For example having for the regressor [0, 0, 1, 1, 2, 2] as output and then using weights=[0, 0, 1, 1, 0, 0] and once 1-weights and check that the expected thing happens.