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

The image display is broken in the HTML rendering of this figure: the name of the example is `plot_calibration_curve.py`, not `plot_calibration_plot`.

Also the target link is broken because of the `/images` component in the target URL.