[MRG] add ranking_loss multilabel ranking metric

This pull request adds the ranking loss. This is a common metric in multi-label ranking task.


Could you use `label_ranking_loss` instead? The name `ranking_loss` is confusing in that it could be instance ranking.

@agramfort Kendall's tau in SciPy is surprisingly slow, even though it's supposed to implement the efficient O(n log(n)) algorithm. In my experience, the dumb O(n^2) algorithm is faster than SciPy (if it's implemented in Cython).
Also the tie handling might be different from what @arjoly wants.