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();
}
```
annotation.events() cannot be null so you could drop the check