bayesian spam evaluation for comment list in admin CP

Adds a statistical ("bayesian") evaluation to comments list in admin CP.

This pull request basically replaces the closed pull request #2 and offers a few enhancements.
* It uses a more recent version of b8 (v0.6 instead of v0.5.2), which has a more compact wordlist format and thus needs less memory/disk space for the word list table. It is also supposed to be a little bit faster as far as accessing the word list table is concerned.
* The database related stuff was rewritten for PDO_MySQL (as opposed to the old mysql API) to work with the current development code.