WIP : Isotonic calibration

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

It's a semantic question (and I'm new so feel free to shut me down :), but I think the base_estimator is the one we are calibrating (and from which the class inherits). This estimator is what we use to postprocess the output of the base_estimator, and I don't think that there are other examples like this to conform to. But it's true that <code>estimator</code> is not very telling, so what if I rename it to <code>calibration_estimator</code>?