[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.
If it works with the 5 lines replacement, +1 for reducing the complexity!
Though, I would replace some summation by dot product if possible or einstein summation.