WIP : Isotonic calibration

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

The scikit is a bit inconsistent in this respect. And to be honest, the classes using `estimator` were there first.

<pre>
$ git grep -l "self.estimator = " .
sklearn/feature_selection/rfe.py
sklearn/grid_search.py
sklearn/multiclass.py
</pre>

<pre>
$ git grep "self.base_estimator = " .
sklearn/ensemble/base.py:        self.base_estimator = base_estimator
sklearn/linear_model/ransac.py:        self.base_estimator = base_estimator
</pre>