function addLoadEvent(func) {
 var oldonload = window.onload;
 if (typeof window.onload != 'function'){
  window.onload = func;
  } else {
  window.onload = function() {
   if (oldonload) {
    oldonload();
   }
   func();
  }
 }
}

function externalLinks() {
 var ls=document.getElementsByTagName('a');
 for (var i=0;i<ls.length;i++) {
  if(ls[i].getAttribute('rel')=='external') {
   ls[i].onclick=function(){window.open(this.href);return false;}
   }
  }
 }

addLoadEvent(externalLinks);