Javascript Library Review.

I am attempting to build a small Javascript Library (similar to jQuery)

The purpose behind this is mostly a learning exercise, so asking why I don't just use an existing library is moot.

So far I have just got my basic constructor, a 'caller' function to invoke a new object, and some basic functions for an example.

Here is the code

var lemon = function() {
    var prop = {
        collection: []
    };
    var fn = {
        init: function(selector, context) {
            context = context || document;
            if (!selector) {
                return;
            } else if (typeof selector == "string") {
                prop.collection = fn.toArray(context.querySelectorAll(selector));
            } else if (selector instanceof Array || selector instanceof NodeList) {
                fn.toArray(selector).forEach(function(obj) {
                    prop.collection.push(obj);
                });
            } else if (selector instanceof Element) {
                prop.collection.push(selector);
            }
            prop.length = prop.collection.length;
            fn.extend.apply(this, [this, prop, fn]);
        },
        toArray: function(target) {
            var array = [];
            for (var i = target.length >>> 0; i--;) {
                array[i] = target[i];
            }
            return array;
        },
        extend: function(target) {
            var objects = Array.prototype.slice.call(arguments, 1);
            objects.forEach(function(obj) {
                var props = Object.getOwnPropertyNames(obj);
                props.forEach(function(key) {
                    target[key] = obj[key];
                });
            });
            return target;
        },
        addClass: function(klass) {
            for (var i = 0, l = this.length; i < l; i++) {
                this.collection[i].className += " " + klass;
            }
            return this;
        }
    };
    fn.init.apply(this, fn.toArray(arguments));
};
var $ = function(selector, context) {
    return new lemon(selector, context);
};