Add support for SQS message attributes

Adds support for [SQS message attributes](http://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/SQSMessageAttributes.html) to both the low level and high level client code, adds a new MessageAttributes object, adds unit tests, updates SQS tutorial with notes and examples of the feature.

@garnaat, @toastdriven any chance of a review?