Allow MessageDispatcher to handle messages based on supplied string name

* Let's say someone publishes messages through SNS to me. The publisher will serialize them with the sns subject set to the full classname of the message definition as seen by the publisher. For example ```com.sun.SomethingHappened```.
* As a reciever of the event, I still want to be able to use the ```MessageDispatcher``` to route messages based on their SNS subject. But I do not want to be forced to make my messages be in the ```com.sun``` namespace. I might want to define them in ```com.rainbowcoloredunicorns.SomethingHappened```.

With this change, you can (as a subscriber), use the ```MessageDispatcher``` for purposes as described above, by calling
```java
messageDispatcher.addHandler(com.rainbowcoloredunicorns.SomethingHappened.class, "com.sun.SomethingHappened", handler);
```