Added support for merchant item filtering by class

Added column classes_required to merchantlist to allow
the merchant to only sell items to specified classes

Ex. (only example I know from live) To only allow poison supplies
to be sell able to only rogues, you would set classes_required
to 256.
classes_required defaults to 65535 (all/any class)

Remember to run the new SQL!
Merge remote-tracking branch 'origin/master' into spellAI