I start with tokens of length 1, and add longer tokens iff they extend a
previously seen token by one character.
Probability computation follow's Paul Graham's "A Plan for Spam", except
that I haven't implemented some of his tweaks (most importantly, I don't
account for frequencs within a message like he does).
While selecting tokens for judging a message, I ignore substrings of
tokens that have been seen previously. This still results in the
majority of tokens to overlap, which is probably not good.
