added ability to handle several events in one method

Sometimes it's useful to have ability to handle several events in one method when you have to react identically to several events. For example you have to reload some grid when user is created or removed. Currently you have to write two methods with the same body:

```
@EventHandler
void onUserCreated(UserCreationEvent e) {
   grid.refresh();
}

@EventHandler
void onUserRemoved(UserRemovedEvent e) {
   grid.refresh();
}
```

My suggestion is to use the following syntax for this case:

```
@EventHandler(events = {UserCreationEvent.class, UserRemovedEvent.class})
void refreshGrid(GenericEvent event) {
   grid.refresh();
}
```
Can you early-exit for the error condition like the previous previous precondition?
e.g.:
if (eventParameter != null && annotation.events().length == 0) {
  logger.log(Type.ERROR, "Method " + method.getName()
            + " annotated with @EventHandler without any 'events' parameter"
            + " must have exactly one argument of a concrete type assignable to GenericEvent");
  throw new UnableToCompleteException();
}