/* Handles the page being scrolled by ensuring the navigation is always in
 * view.
 */
function handleScroll(){
  // check that this is a relatively modern browser
  if (window.XMLHttpRequest){
    // determine the distance scrolled down the page
    var offset = window.pageYOffset ? window.pageYOffset : document.documentElement.scrollTop;
    // set the appropriate class on the navigation
    document.getElementById('navigation').className = (offset > 0 ? 'fixed' : '');
  }
}

// add the scroll event listener
if (window.addEventListener){
  window.addEventListener('scroll', handleScroll, false);
}else{
  window.attachEvent('onscroll', handleScroll);
}