[MRG] Multi-output scoring, or 2493 cont'd for real

This time taking into account the refactoring of metrics.py into several files.

This replaces https://github.com/scikit-learn/scikit-learn/pull/3456, which I closed.
Not sure it renders properly in the doc.

Maybe this would be shorter
`array-like of shape = [n_outputs], ['uniform', 'variance' or None (default)]`