XD 647 - Enhanced Message Conversion using content-type headers

New converter package based on Spring 4 MessageConverter and recent updates to SI Data Type channel. Converters have access to the entire message so may process the content-type header.  

- A major change is the http source now emits byte[] by default instead of always a String. Tests have been updated using 

            http --outputType=java.lang.String

Note, in previous versions --outputType=text/plain would convert byte[] payload to String. This is no longer the case as the content-type text/plain is also valid for byte[], so better to be specific about converting to String. There is also an application/x-xd-string  content type as an alternative to using FQ classname.  Documentation and Demo's will need to be updated to reflect these changes.