[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.
pep8 addressed. Don't tell me to use einsum - I love einsum, and I'll put it everywhere if you encourage me too much. One day later I'll try to read the code and say " 'ij, i, j' ... wait ... what?"