Attempt to allow null and empty country objects

Possible fix for #82. I'm not totally certain I got everything right, as we might want to add more tests to verify behavior is working appropriately. The biggest thing to check here might be the round-tripping of both `None` and `u''` values, to make sure the code attribute is set appropriately on the returned Country object from the descriptor.
Hrm, maybe we should just change the __eq__ test to `return force_text(self or '') == force_text(other or '')` then just extend the tests to check for `''`, `None` and `fields.Country(None)`