COSMIT Typos fixed in plot_calibration.py
DOC Revised calibration.rst based on ogridels suggestions

 * explaining what well calibrated classifiers 
 * links fixed
 * reference added
REFACTOR CalibratedClassifier made private; use CalibratedClassifierCV with cv="prefit" instead
TEST Added test for calibration with prefitted classifier.
REFACTOR Using bincount in calibration_curve()
ADD Example comparing the calibration curves of different classifiers on toy dataset
DOC Extended discussion of calibration in narrative doc
REFACTOR Rename _check_and_normalize() to _check_binary_probabilistic_predictions()