Add SQS listener in adapter

As `lita-slack-adapter` needs to expose robot to internet to let out outgoing webhook hit robot, and as Lita does not suppor multiple adapters yet, I've written a SQS module to subscribe from a SQS queue to feed robot with messages in the queue.

`aws-sdk` is loaded when `sqs_queue_name` is declared, and also there are new optional config variables to work with SQS.