Add Multinomial Naive Bayes classifier

Naive Bayes is a simple classifier that is able to be updated in a
streaming fashion (i.e. it can be updated with each new instance rather
than having to iterate over the entire training set each time).
Multinomial Naive Bayes uses word counts rather than a binary present or
not-present value. This closes gh-3.

Note that currently use the `Fraction` class to compute the *exact*
probability. It might be more space and time efficient to use [log
probability](https://en.wikipedia.org/wiki/Log_probability) instead.
However, for small document lengths, such as we have with tweets, the
slower time computing the exact may not matter.