Make it an Event Dispatcher decorator.

This would definitely be a BC break. I'm happy to work around it some more if you want something changed. Let me know what you want. :)

All of the tests should still pass. (Works on my machine) I was using [this version in Depot](https://github.com/depot/depot/blob/master/src/Depot/Api/Server/Symfony/EventDispatcher/PimpleContainerAwareEventDispatcher.php) but would love to just have it submitted here.