[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.
The 'expected thing' would need to be manually computed for each different scorer, right?

That would be just like coming up with a different hand-crafted test for each scorer, which I could do, if you insist.