Moving TargetedEventInterface implementation from trait into TargetedEvent class

The `TargetedEventTrait` was attempting to implement an interface; PHP does not allow this:

```
Fatal error: Cannot use 'TargetedEventInterface' as interface on 'Phergie\Event\TargetedEventTrait' since it is a Trait
```

It could be removed, and `TargetedEvent` could implement `TargetedEventInterface` instead, but I don't see any real benefit to using traits here - anyone wishing to utilise implemented `TargetedEventInterface` behaviour can simply extend `TargetedEvent`.