QPID-4567: Removed errors from qpid_messaging.rb.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1444113 13f79535-47bb-0310-9956-ffa450edef68
QPID-4573: Add a dispatcher for Sessions in Ruby.

The dispatcher runs in the background, surrendering the global
interpreter lock in Ruby. It then waits for the session to have a
receiver with a pending message to be processed. When such happens it
wakes up and notifies the session.