Add event retrieval helper to dispatcher

[joeholdcroft]

When dispatching an event, the event object must be passed in to the dispatcher. Often we want to get this out of the service container. So it makes sense to add a helper to the dispatcher to do this.

$dispatcher->dispatch(
    'eventname',
    $dispatcher->get('myevent')
);

Passing argument 1 as 'myevent' would return event.myevent from the service container. Passing nothing would return the event defined as event on the service container.

The second argument would be an array of arguments to pass to the constructor.