Add support for Foo__isnull=True/False. Also, clean up our generate_param and get_filter_params to only use locally scoped variables for code clarity.
Ensure that filter always returns a set of results, even if only a single result is returned from the API. This brings pyxero inline with the Django ORM.
Leave the undecorated method behind and available via an underscore (e.g. _filter).
Ensure that now with the XML being parsed correctly, we return a single instance for Manager.get, and update tests accordingly.
Merge in pull request #41 from @gavinhodge (https://github.com/freakboy3742/pyxero/pull/41)