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.
The idea here was that force_text() None returns the quoted string "None", which is unfortunate, and str() has the None-check code. I'll revise it to make it 2/3 compatible.