Using sabre/event for event management

Hi!

The attached pull request migrates from using an internal event management system to using [sabre/event][1].
It has a bit more options, but still has the same feature set; and the method signatures are nearly identical.

The main differences are:

  * If an event handler (subscribed with `on`) returns a literal `false`, event handling stops.
  * There is no `buildParameters`, but it's easy to add this feature anyway.
  * The default priority is 100 instead of 10, so I multiplied every priority with 10.

I hope you like it. I realize this is a selfish PR as I'd like to see my library used more. I would be willing to add `EmitterAwareInterface` and `EmitterAwareTrait` to sabre/event, if that helps convincing you :)

[1]: https://github.com/fruux/sabre-event