* This acts as a controller for firing events. For example if we have the
* following <code>Event</code> object heirachy:
*
* <pre>
* ClientEvent extends Event
* LoginEvent extends ClientEvent
* LogoutEvent extends ClientEvent
* </pre>
*
* Then add the following <code>EventListener</code> to the
* <code>EventManager</code>:
*
* <pre>
* addListener(LoginEvent.class, new EventListener() {
* public void eventFired(Event e) {
* System.out.println(e.getClass().getName());
* }
* });
* </pre>
*
* The event listener will only be called when <code>fireEvent</code> fires a
* new <code>LoginEvent</code> and the output will always be 'LoginEvent'.
* However if we made the listener listen for
* <code>addListener(ClientEvent.class,</code>... the event listener will be
* called for every <code>LoginEvent</code>, <code>LogoutEvent</code> and
* <code>ClientEvent</code> making the output either 'LoginEvent',
* 'LogoutEvent' or 'ClientEvent'. Ideally each <code>Event</code> subclass
* should be immutable so that all event listeners are guaranteed to receive the
* same state.