Move const variable into class definition?
In #347 you mentioned redoing the connection interface. Just thought I'd mention this...

@cboden If/When you get around to modifying the connection interface, you might also move the [const variable](https://github.com/ratchetphp/Ratchet/blob/master/src/Ratchet/ConnectionInterface.php#L8) into the interface declaration. It pollutes the global namespace otherwise. Notice the attached screenshot the ratchet\VERSION global variables. While it is namespaced, it still shows up every time my debugger breaks. Minor, but would be appreciated. (Not sure if "global namespace" is the right term, but I hope you get what I'm trying to explain.)

![screenshot from 2016-04-24 11-11-41](https://cloud.githubusercontent.com/assets/1414400/14769477/d92e07e2-0a0d-11e6-84e2-cf5dbcb5d5c0.png)
