Fixes enumerator ending

Fixes #28. Previously, the Enumerator would end when SQS did not have a message. This behavior was not intended, so has been changed so the `Enumerator` will produce (sans exception) for as long as it is being read from by an `Iteratee`.

This is an API compatible change, but changes behavior. To upgrade, ensure that you are not relying on the Enumerator ending when the queue is empty. If you want that behavior, use `next` instead.