SQS Output Now Supports Batching

* batching of messages to SQS via Stud::Buffer
* default settings result in a 90% cost savings under heavy load
* batching is the default, but may be disabled with config paramter batch set to false
* batch_events config parameter determines the max number of messages per batch.  Valid values are 2..10.
* batch_timeout config parameter determines the max time to wait before sending a partially full batch.