Db\Sql\Update add sortable set list

for compatibility with http://dev.mysql.com/doc/refman/5.0/en/ansi-diff-update.html.
Because sometime set list should be reordable.

Moved ```Zend\Mvc\Router\PriorityList``` to ```Zend\Stdlib\PriorityList``` because ```Db\Sql\Update``` require it and StdLib is more logical location.