jQuery: parallax scrolling with the mousewheel event

HTML

<div id="site">
  <div id="section-wrapper">
    <div class="section" id="section-1">111</div>
    <div class="section" id="section-2">222</div>
    <div class="section" id="section-3">333</div>
  </div>
</div>

CSS

html, body {
    overflow: hidden;
}

#site {
    width: 100%;
}

#section-wrapper {
    position: relative;
    width: 100%;
}

div.section {
    width: 100%;
    position: relative;
}

#section-1 {
    background: #ccc;
}

#section-2 {
    background: #666;
}

#section-3 {
    background: #000;
}

JavaScript

(function($){var types=['DOMMouseScroll','mousewheel'];if($.event.fixHooks){for(var i=types.length;i;){$.event.fixHooks[types[--i]]=$.event.mouseHooks;}}
$.event.special.mousewheel={setup:function(){if(this.addEventListener){for(var i=types.length;i;){this.addEventListener(types[--i],handler,false);}}else{this.onmousewheel=handler;}},teardown:function(){if(this.removeEventListener){for(var i=types.length;i;){this.removeEventListener(types[--i],handler,false);}}else{this.onmousewheel=null;}}};$.fn.extend({mousewheel:function(fn){return fn?this.bind("mousewheel",fn):this.trigger("mousewheel");},unmousewheel:function(fn){return this.unbind("mousewheel",fn);}});function handler(event){var orgEvent=event||window.event,args=[].slice.call(arguments,1),delta=0,returnValue=true,deltaX=0,deltaY=0;event=$.event.fix(orgEvent);event.type="mousewheel";if(orgEvent.wheelDelta){delta=orgEvent.wheelDelta/120;}
if(orgEvent.detail){delta=-orgEvent.detail/3;}
deltaY=delta;if(orgEvent.axis!==undefined&&orgEvent.axis===orgEvent.HORIZONTAL_AXIS){deltaY=0;deltaX=-1*delta;}
if(orgEvent.wheelDeltaY!==undefined){deltaY=orgEvent.wheelDeltaY/120;}
if(orgEvent.wheelDeltaX!==undefined){deltaX=-1*orgEvent.wheelDeltaX/120;}
args.unshift(event,delta,deltaX,deltaY);return($.event.dispatch||$.event.handle).apply(this,args);}})(jQuery);

var Parallax = {
    utils: {
        doSlide: function(section) {
            var top = section.position().top;
            $('#section-wrapper').stop().animate({
                top: -top
            }, 600, 'linear', function() {
                section.addClass('slided').siblings('div.section').removeClass('slided');
            });
        }
    },
    fn: {
        setHeights: function() {
            $('div.section').height($(window).height());
        },
        onSiteScroll: function() {
            var section = null;

            $('#section-wrapper').mousewheel(function(event, delta) {
                event.preventDefault();
                if (delta < 0)...