function externalLinks() {
   if (!document.getElementsByTagName)
   {
      return;
   }
   var anchors = document.getElementsByTagName("a");
   for (var i=0; i<anchors.length; i++) {
      var anchor = anchors[i];
      if (anchor.getAttribute("href") && anchor.getAttribute("rel") == "external")
      {
         anchor.target = "_blank";
      }
   }
}

function addEventHandlerToChain(object, event, theFunction)
{
   var prev = object[event];
   object[event] = function(){ if(prev)prev(); theFunction(); }
}

addEventHandlerToChain(window, "onload", externalLinks);