A few ideas (do not know if they are usable for others)

[vakata]

You could probably return the animation frame (or timeout) ID, so that it can be cleared later (maybe we want to ignore quick subsequent scroll events and only fire once):

        requestAnimFrame = (function () {
            return window.requestAnimationFrame ||
                window.webkitRequestAnimationFrame ||
                window.mozRequestAnimationFrame ||
                function (callback) {
                    return window.setTimeout(callback, 1000 / 60);
                };
        }()),
        cancelAnimFrame = (function () {
            return window.cancelAnimationFrame ||
                window.webkitCancelAnimationFrame ||
                window.mozCancelAnimationFrame ||
                function (id) {
                    window.clearTimeout(id);
                };
        }()),

That would mean we should store the ID per element so that we can clear it:

        if (this._collection[el] === undefined) {
            this._collection[el] = {
                'id' : false,
                'listeners': []
            };

So that when the event fires we can clear the previous frame (timeout):

            /**
             * requestAnimationFrame
             */
            if(scroll._collection[scrolledElement].id) {
                cancelAnimFrame(scroll._collection[scrolledElement].id);
            }
            scroll._collection[scrolledElement].id = req...

These are just off the top of my head (sorry if I missed something and this is not a valid idea), also sorry for the bucket of code - if you think this is usable, but the code here is too messy, I can create a pull request.

Cheers!