Added interface to listen to client messages.

src/rosjava_actionlib/rosjava_actionlib/src/main/java/com/github/ekumen/rosjava_actionlib/ActionClientListener.java

+package com.github.ekumen.rosjava_actionlib;
+
+import org.ros.internal.message.Message;
+
+/**
+ * Listener interface to receive the incoming messages from the ActionLib server.
+ */
+public interface ActionClientListener {
+  void resultReceived(Message result);
+  void feedbackReceived(Message feedback);
+  void statusReceived(Message status);
+}