Add base of the bayes plugin.
Add base of the bayes plugin.

- There is still significant work to be done here.
- This is a reimplementation of essentially all of the SA code,
keeping it as unchanged as possible. Since we want to have database
compatibility as much as possible, we need to keep the tokenisation
and probability generation as identical as possible.
- Where there are variations, those are noted in the code.
- Docstrings are still missing.
- There are a few bits of Perl that still need to be converted.
- There are no timers, which are used extensively in SA. In most
places that use timers, there's a comment in the code.
- Proper hooking to the OA plug-in system still needs to be done.
- Added notes to the combining system comparing it to SpamBayes, for
reference.
SpamExperts/OrangeAssassin