[MRG+1] 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.
It does seem to me `_average_and_variance` is used only twice below right? I just did `git grep _average_and_variance` and it returns two results.