Correct the docs and examples regarding the message ID property

Author: ssorj

README.md

@@ -406,7 +406,7 @@ Sends a message. A message is an object that may contain the following fields:
   * content_type
   * content_encoding
   * group_id
-  * id
+  * message_id
   * correlation_id
   * application_properties, an object/map which can take arbitrary, application defined named values
   * body, which can be either a string, an object or a buffer

examples/direct_send.js

@@ -29,7 +29,7 @@ container.on('sendable', function (context) {
     while (context.sender.sendable() && sent < total) {
         sent++;
         console.log('sent ' + sent);
-        context.sender.send({id:sent, body:{'sequence':sent}});
+        context.sender.send({message_id:sent, body:{'sequence':sent}});
     }
     if (sent === total) {
         context.sender.set_drained(sent === total);