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.
Just a thought to discuss tomorrow...if the xml schema for a CA changes, would it be good to be able trap malformed xml type errors and turn them into SymantecMalformedResponse errors or some such? Just thinking of the logging of errors on the Barbican side.