Add initial SQS support and decent HIT and Assignment maps

This includes the `activate-sqs-notification` function, which complies and runs. However, I'm still setting up my SQS queue, so I haven't truly tested it end-to-end with SQS yet.

I also filled out the HIT and Assignment mappings; they are basically complete.

Relevant:
http://docs.aws.amazon.com/AWSMechTurk/latest/AWSMturkAPI/ApiReference_NotificationDataStructureArticle.html
http://docs.aws.amazon.com/AWSMechTurk/latest/AWSMturkAPI/ApiReference_NotificationReceptorAPI_SQSTransportArticle.html

Also, please see TODOs in project.clj.