adapt schema parser to the new XmlObject api
fix variable creep from upper scope
fix open() calls for python3
remove attribute_of
make protocol.msgpack handle large requests from twisted which comes in as mmap()'d objects #402
Model documentation

* add get_documentation classmethod
* make sure documentation is kept through customize calls
* default __use_parent_doc__ to True
* prefer cls.Annotations.doc to cls.__doc__