BBCode Additions

I've been making additions to BBCode. Support has been added for header tags, pre, and tables. The autop (insert linebreaks) and texturize (special symbols) methods are run with BBCode. Initially I had them run after the BBCode filter, but that seemed to cause some problems with the center tag (and possibly others). Now they run first, which works, but is a bit gratuitous in its insertion of linebreaks.