[MRG] Add multilabel support to precision, recall, fscore and classification report

This pull request adds support for mulltilabel output support for the following functions:

-   classification_report
-   f1_score
-   fbeta_score
-   precision_recall_fscore_support
-   precision_score
-   recall_score

One more pull request to tackle the issue #558.

Two questions:
 - What should we do when the computation of the precision, recall or fscore lead to a nan? Currently if the computation lead to a `nan`, the related  measure is set to `0.0`?
 - Which tests should I change to use the new precision, recall  and fscore function?