title: Add / Remove Class when scroll to an ID in the mid
body: <p>I have a website in twitter bootstrap with top fixed navigation.<br>
  I have two navigations, one is on top and other one is in mid. Top one have a fixed position but requirements are when we scroll to mid and reached to mid navigation, add fixed class to mid navigation and remove fixed class from top Navigation.</p>
  
  <p>Thanks for help in advance.
  Murtaza</p>