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.
Yea I was thinking about something along those lines, I had talked to Sheena and I think she was going to make a separate story for that. I wasn't aware of how the responses would look so I left it pretty free form until we saw patterns emerge.