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

This pull request adds support for mulltilabel output support for the following function :+1: 


One more pull request to tackle issue #558.

Reviews and comments are welcome! :-)

Two question:
 - What to 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?