[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 couldn't find it for scorers, just for metrics.


On Thu, Jul 17, 2014 at 2:38 PM, Andreas Mueller <notifications@github.com>
wrote:

> In sklearn/metrics/tests/test_score_objects.py:
>
> > @@ -25,6 +29,13 @@
> >  from sklearn.multiclass import OneVsRestClassifier
> >
> >
> > +REGRESSION_SCORERS = ['r2', 'mean_absolute_error', 'mean_squared_error']
>
> is that not defined somewhere else?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/scikit-learn/scikit-learn/pull/3401/files#r15054057>.
>