[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.
support for Py2.6 means we can't use `{}`. Use `{0}`, `{1}`, `{2}`, ...