Add new callback, onCollectionAddCallback

* Added a callback to handle actions after an element was added to the collection.
* Remove unused global variable
Instead of `onCollectionAddCallback != undefined` I think you should use Coffeescript's existential operator: `onCollectionAddCallback?`. Also, I don't think we should limit this functionality with `and @onCollectionAddCallback is null`; if the application already specified a callback, then we should just run

````coffeescript
@stream.collection.off 'add', @onCollectionAddCallback
@onCollectionAddCallback = onCollectionAddCallback
@stream.collection.on 'add', @onCollectionAddCallback
````

Finally, I am not sold on using a callback for this functionality. What f instead of accepting a callback the stream exposed a promise? That way an application could attach any amount of listeners they want. Could be an idea.