Implement better handling of contentType including adding a mechanism to serialise and deserialise native JavaScript Objects as JSON as an alternative to using the AMQP type system. The contentType handling also transparently converts text MIME types to String on the received message

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/branches/fadams-javascript-binding@1624841 13f79535-47bb-0310-9956-ffa450edef68