WIP simplify naive Bayes parametrization

After moving ``class_prior`` to ``class_weight`` as in ``__init__`` parameter in #1499, the parametrization was a bit weird. I now got rid of ``fit_prior`` and let ``class_weight`` be ``'auto'`` ,``None``(=uniform) or an array. This is a bit more consistent with the other estimators.

I'll try to make it pass the common tests now and I guess I'll also allow dictionaries.