WIP : Isotonic calibration

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

>
> Why two examples? And they have hard to distinguish names...
>
> see now the narrative doc. One example on a toy really explains visually
the problem of calibration while the other one uses covertype to show a
real world example. What headline
would you recommend for both examples?