[MRG] partial_fit for the discrete naive Bayes models

Early PR to review an implementation of `partial_fit` for the discrete naive Bayes models.

TODO:

- [x] check test coverage and add more tests if necessary
- [x] check on a non-toy datasets such as the out-of-core Reuters example
- [x] identify ways to factorize common code
- [x] update documentation to mention the new method where applicable
- [x] fix broken test: a rebase on master highlighted a regression introduced in this PR

Note generic topical documentation for out-of-core learning in general will be dealt with in #2204.