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.
Continuing Donald's comments above...this test would need to go into a tests/test_utils.py module or what not.