WIP : Isotonic calibration

calibration module with platt and Isotonic calibration.

A few issues :

- [ ]  nice calibration plots
- [ ]  add tests for metrics
- [ ]  add narrative doc

>
> Have you compared with just averaging the regular probabilities? (not the log-probabilities)
>
> I did not but averaging proba is not really recommended in general. If you
have time to take over please do. I get caught by other obligations :(