Implemented Revoke, and updated GetOrderByPartnerOrderID

GetOrderByPartnerOrderID should now return all associated metadata with the order. I wanted to get this in now, but am aware that I will need to rebase after Paul G's latest PR lands. Also implemented a function, xml_to_dict, that could be useful. Let me know if it should be stored somewhere instead of order.py or if it is just unnecessary.
This would be another good story I think. I could think of a few ways of doing it, but it should probably live on BaseModel with a schema defined decoratively on the subclasses.