[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 output_weights:` will lead to an error saying "The truth value of an array is ambiguous. Please use np.all [...] instead", if an array is passed.