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.