Artanys

Will fix tests, pushed this out a bit late last night :) I modeled the group name after the implementation of xs:choice, with the addition that xs:all is a direct...

```xml ``` gets generated if I specify something like ```python class AbcAll(ComplexModel): abc = Unicode(xml_all_group="all1", min_occurs=1) bcd = Unicode(xml_all_group="all1") cde = Unicode(xml_all_group="all2") ``` This then fails lxml schema validation in...

Schema says they cannot coexist. Will look into making it a class attribute