PROTON-385: Added the body property to Perl Message class.

Added the preencode and postdecode methods to Message, with appropriate
calls from Messenger, to encode and decode the body itself.

The send.pl and recv.pl examples now use the body type, with send.pl
sending random values as an example for using the body field.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1520109 13f79535-47bb-0310-9956-ffa450edef68