[MRG]  mean absolute error and multioutput regression metrics

This pull request implement two new features :
 * add the metrics `mean_absolute_error`;
 * add multioutput support for `r2_score`,  `mean_absolute_error` and `mean_square_error`.

I have been unable to find a narrative documentation corresponding to those regression metrics. So I wrote it. 

Reviews and comments are welcome. :-)