Add optional sqs queue output

adds `sqs_queue_name` and `sqs_endpoint` options; if `sqs_queue_name` is specified file deliveries to S3 will also result in a message to SQS indicating the bucket and path of the file delivered. 

we wanted to have a simple way to know which S3 files have been processed downstream after fluentd drops them off to S3, this avoids having to do any sort of pipeline high-water-marking and allows our consumer to just pull files to process off of the SQS queue (without having every single event be it's own item in the queue, as is possible with https://github.com/ixixi/fluent-plugin-sqs