Hydrator aware interface

Hi.

I've created a HydratorAwareInterface, that mimics the behaviour of the InputFilterAwareInterface.

This way we can avoid setting the hydrator on the form every time, we just make the object that we bind to the form aware of its own hydrator (with a simple getter and setter).

The patch uses the object hydrator if the form doesn't have one hydrator already, and this might be one of the points were you for sure have a better understanding if this is correct or not.

Thanks,
Gonçalo Queirós