title: jquery - carry out function after scroll past x pixels
body: <p>I've made a nav on my site which is absolute positioned.</p>
  
  <p>I've made a class to make this fixed to the top of the screen.</p>
  
  <p>What I'm trying to find out is how to carry out a function (toggleClass in this instance) after the windows has been scrolled x amount of pixels down the page (500 px in this instance)</p>