changed setFormHydrator parameter type

Using AbstractHydrator is possible to use other
zfcuser_register_form_hydrator for example:
DoctrineModule\Stdlib\Hydrator\DoctrineObject
Use of HydratorInterface in setFormHydrator and getFormHydrator
Merge pull request #314 from stefanorg/master

Ability to use different kind of hydrator with user registration form:  zfcuser_register_form_hydrator