added paralax scrolling to adi website

as shown below, the background image move slower than main.
with parallax scrolling
![screen shot 2014-12-05 at 2 02 11 pm](https://cloud.githubusercontent.com/assets/8745781/5321260/8a33aff8-7c87-11e4-9edc-5411f2e9df82.png)
![screen shot 2014-12-05 at 2 02 32 pm](https://cloud.githubusercontent.com/assets/8745781/5321261/8a38632c-7c87-11e4-8176-097425365c76.png)
vs
without
![screen shot 2014-12-05 at 2 04 55 pm](https://cloud.githubusercontent.com/assets/8745781/5321279/bd76b536-7c87-11e4-9353-d595663afc49.png)

This is where magic happens: dividing scrolled by 2 and assign it to margin-top
    $(window).scroll(function(e){
      parallax();
    });
    function parallax(){
      var scrolled = $(window).scrollTop();
      $('.hero i').css('margin-top',(scrolled/2)+'px');
    }