Fix issue with parsing MWS response containing member lists

I've just started working on MWS using boto for the API communication and realised that parsing some of the responses provided in the Amazon MWS documentation cause the response parsing to break. I am not sure if the test case I added including the inline XML is the way you'd like that sort of data to be arrange. Please let me know if you want me to re-arrange the test and sample data.

The initial problem by the response was causing an error with ``issubclass`` because ``self._hint`` was an instance rather than a class. Since ``isinstance`` handles classes *and* instances and is aware of inheritance, this change should be sensible and hasn't caused any other issues.

A second issue I ran into was that ``delattr(self._parent, self._name)`` would raise an ``AttributeError``. Although ``hasattr(self._parent, self._name)`` is true, it is not in the object namespace for ``self._name = member`` and cannot be deleted. I am not sure if this is the right way to solve this problem but am not aware of a better solution. Happy to be educated on a better way and update the PR.

Let me know if you have any questions or would like me to make changes.