Method to return sqs message body as a hash

A lot of times, an SQS message body will contain structured JSON data.  For instance, when Auto-Scale sends a scaling activity notification via an SQS queue, the message body (stored internally as data[:body]["Message"]) is in JSON.   

Unfortunately, the current methods only return this JSON as a string.   Every message then has to be parsed.  This seems like a pretty common use case.   

This pull request adds that functionality through a method called body_message_as_h.  This name is consistent with the internal data structure data[:body]["Message"].