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?
The paper "A note on Platt's probabilistic outputs for support vector machines" might be worth reading. It describes the technique used in libsvm. I haven't checked if this differs from the code below though.