WIP : Isotonic calibration

working version of calibration module containing Isotonic calibration.

I'd like to have feedback on API

A few issues : brier_score is not a score (cc @mblondel)
I am not sure of how to handle OOB samples

@mblondel : any idea of how to handle the multiclass case?
I still think we should support multiclass classification with one-vs-rest "out of the box".
In " Transforming Classifier Scores into Accurate Multiclass", the authors recommend just normalizing the probabilities.