Add new class "MessageQueueListener"

It is hard to extend interface class because all impelementation
classes are affected.

To avoid such situation, new abstract class is added. In the same time,
existing interface class is being deprecated. (no longer notified)
Pass headers in message property to onReceive()
Move class MessageQueueListener to org.jenkinsci.plugins.rabbitmqconsumer.extensions
Adjust package location
Add new class MessagePublisher
Update README
Remove unnecessary comparation for overrided equal() and hashCode()
[maven-release-plugin] prepare release rabbitmq-consumer-2.0