Track ticket system - spam filtering

Hi,

we've had multiple false positives recently with our spam filter.
The system should never get in the way of legitimate users, so I went
and tweaked the settings based on what I saw in the logs.

It should now be less aggressive in refusing submissions that contain
code snippets or XML/HTML. Those tend to score higher on the bayesian
filter, apparently due to similarity to the spam HTML. As spam tends to
score much higher than those false positives, I've rebiased the scoring
to account for that. There might be some false negatives in the near
future, so if you see spam on trac, please let me know directly.

Cheers

Thomas