Python 3 metaclass compatibility for Field subclasses.

This fixes fields' Python 3 support.

If you're using ``JSONField`` in Python 3, ``to_python`` is never called because the subclass is using Python 2's ``__metaclass__`` syntax.

I also changed ``EncryptedTextField`` and ``EncryptedCharField`` for future compatibility -- it doesn't seem python-keyczar supports Python 3 (at least the tests don't run).