Access SQS with minimum amount of priveleges

When the queue is specified by URL, only the following permissions are needed: `sqs:DeleteMessage`, `sqs:ReceiveMessage`

## Details
If the queue is specified by URL, no high-privilege AWS actions are taken to check for
the queue existence, or to create the queue in case it doesn't exist. If you've set things up with very limited AWS permissions, the plugin will work fine as long as you specified correct credentials and queue URL.