title: Jquery add active class depending on scroll
body: <p>i am creating one page website with scroll to plugin. I want to add active class to menu items depending on how far i scroll.</p>
  
  <p>Navigation: About us | Services | Prices | Contacts</p>
  
  <p>if i scroll from 0 to 800 pixels from top than About us will be active,
  if i scroll from 800 to 1600 pixels from top than Services will be active</p>
  
  <p>and so on...</p>
  
  <p>Thanx</p>