Generic resource hydration

I added ``full_hydrate`` and ``full_dehydrate`` methods to ``GenericResource``.
So now you can create/update ``GenericForeignKeyField``'s objects with POST/ PATCH methods.

I deleted `` test_get_related_resource`` becouse `` test_get_related_resource`` is returning a ``GenericResource``  instance now.
All logic for choosing resource implements in ``GenericResource``  class.

Finally i added ``ContentTypeResourceMixin`` to ``GenericResource`` for content_type output.