[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.
nonzero_numerator * nonzero_denominator => nonzero_numerator & nonzero_denominator?
Or you could use np.logical_and.