[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.
This text is almost identical for explained_variance_score, r2_score, mean_absolute_error and mean_squared_error (missing in narrative doc). What do you think of creating a separate section to explain the output_weight (and maybe sample_weight) argument?