Add Support for Message Attributes

SQS now includes the ability to include `Message Attributes` when posting messages to queue.  The complete AWS documentation is available [here](http://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/SQSMessageAttributes.html#SQSMessageAttributesNTV).  This pull request implements base support for message attributes.  The largest current restriction is that we only accept attributes of String type.  All test cases should be passing with these modifications.