Derives an interface from AbstractEvent

Adds an interface based on previous discussions.

Notes:
* Since the interface is derrived from `AbstractEvent` every event is there. I am not sure if set/getEmmitter should as well, but didn't check any code, etc, so might not be right.
* I updated docs/spec where it was possible, however (since my lack of understanding in phpspec) it should be reviewed as well.
* Added it to the readme without version/release date data.

Let me know if I missed something.