[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.
I tried putting it like that, but it goes way over 80 chars in that line. What is the issue for the docs? The linebreak?