[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.

you don't like pairwise_ranking_loss ?

> @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.

ok then scipy should be fixed... at some point ...