_.extend = function(target) {
        var i = 1, length = arguments.length, source;
        for ( ; i < length; i++ ) {
            // Only deal with defined values
            if ( (source = arguments[i]) !== undefined ) {
                Object.getOwnPropertyNames(source).forEach(function(k){
                    var d = Object.getOwnPropertyDescriptor(source, k) || {value:source[k]};
                    if (d.get) {
                        target.__defineGetter__(k, d.get);
                        if (d.set) target.__defineSetter__(k, d.set);
                    }
                    else if (target !== d.value) {
                        target[k] = d.value;
                    }
                });
            }
        }
        return target;
    }