add UniqueRequestToken option to mturk connection CreateHIT API

http://docs.aws.amazon.com/AWSMechTurk/2012-03-25/AWSMturkAPI/ApiReference_CreateHITOperation.html

"UniqueRequestToken
A unique identifier for this request. Allows you to retry the call on error without creating duplicate HITs."

Havn't been able to test this since running `python tests/test.py` seemed to stall forever (see below), but it's a simple change so it should just require a test run to make sure everything goes smoothly.