Stdlib - PriorityList move from Zend\Mvc\Router to Stdlib

```Zend\Mvc\Router\PriorityList``` exist in ```Zend\Mvc\Router``` namespace.
But it also required for :
 - ```Zend\Form\Fieldset``` : it can be used for ```$iterator``` property, and ```$byName``` can be deleted (now used both variables). 
 - Also if using ```foreach(Form\Fieldset::getIterator() as $name=> $element)``` the ```$name``` should be element name instead of index (as the ```Form\Fieldset::getElements()```)
 - ```Db\Sql\Update``` : Because sometime the values set should be reordable (http://dev.mysql.com/doc/refman/5.0/en/ansi-diff-update.html)
 - perhaps somewhere else can be used