Add ability to register a custom hydrator as a class instance instead of a class name

Currently its only possible to register a custom hydrator as a string name. However in order to inject additional attributes it would be beneficial to be able to register an instance of the custom hydrator class.

http://www.doctrine-project.org/documentation/manual/1_2/en/data-hydrators#writing-hydration-method

So instead of:
$manager->registerHydrator('my_hydrator', 'Doctrine_Hydrator_MyHydrator');

One could do:
$manager->registerHydrator('my_hydrator', new Doctrine_Hydrator_MyHydrator());