Feature - Message attributes

This PR introduces [Message Attributes][aws-sqs-message-attributes] to the SQS package.

This PR implements the necessary models, the necessary request parameters, and tests. Although I would have loved to, this PR does not include the _sending_ of messages with "Message Attributes", as it would have caused an API change.

If we could decide on a sane API change, I would gladly produce a PR for that functionality.

[aws-sqs-message-attributes]: http://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/SQSMessageAttributes.html