WIP : Isotonic calibration

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

`np.unique(y) == [1, 2]` which is fine but then it should be made explicit which is the positive label (e.g. `pos_label=2`) and passed explicitly as a parameter to the `brier_score_loss` function and the `CalibratedClassifierCV` class. Otherwise it's magic.