Add ability to notify SNS topics and SQS queues when downloading

[cameronhunter]

Instead of polling the status of a retrieval action such as download, allow the user to specify a SNS topic to be notified on on completion. This goes hand-in-hand with specifying a SQS queue with the actual data to be retrieved.

This feature should be an option to the user, but not the default. It is much simpler for the user to simply ask for a download and it just poll until completed.