WIP : Isotonic calibration

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

I think the project should really not create its own terminology... Maybe `brier_score_loss` if you insist?

Also this is just a naming problem: there's is no ambiguity in the scorer API thanks to `greater_is_better`.