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

I would split this paragraph (just before "Two approaches...") and insert here a definition of what is a well calibrated model. Here is my attempt at such a definition:

Well calibrated classifiers are probabilistic classifiers for which the output of the `predict_proba` method can be directly interpreted as a confidence level. For instance a well calibrated classifier should on average predict the right class 80% of the time for all the samples for which it gave a `predict_proba` value close to 0.8.