add support for stream end

Hi,
this patch add support for ending stream.
publishing kamo.End the subscriber is executed and after deleted from stream subscriptions.

```
    var stream = new kamo.Stream();
    var log = function(value) {
      console.log(value);
    };
  
    stream.subscribe(log);
    stream.subscribe(log);
    stream.subscribe(log);
    stream.publish("ciao");
  
    stream.publish(kamo.End);
   //this last subscriber isn't executed 
   stream.publish("ciao2");
```