Add partial_fit to GaussianNB

Uses a [method due to Chan, Golub, and LeVeque](http://i.stanford.edu/pub/cstr/reports/cs/tr/79/773/CS-TR-79-773.pdf) to perform online update of the model parameters in `GaussianNB`. Does not implement sample weighting.

Added tests to `test_naive_bayes` to ensure that `partial_fit` called on the entire toy set produces the same result as `fit`, and that it produces the same result even if the toy set is fitted in two parts.

TODO: `fit` could be a thin wrapper around `partial_fit` rather than duplicating code.
> We use exclusive float64 (and sometimes exclusive float32) for model
> parameters in lots of places. If we don't, porting Python code to
> Cython becomes very complicated.

I agree. I just wanted to stress that this is a status quo for now, but
not something that we are terribly happy with.