dbasebox: added updating existing tables (not finished)
dbasebox: removed 'marked' field on module uninstall

The fields ('marled') added to existing tables ('node') by a module
(dbasebox) by db_add_field are not automatically removed on module
uninstall and have to be removed manually by calling db_drop_field()
in hook_uninstall().
dbasebox: implemented hook_query_alter()