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

//addLoadEvent(nameOfSomeFunctionToRunOnPageLoad);
//addLoadEvent(function() {
  /* more code to run on page load */ 
//});

addLoadEvent(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";
anchor.title = (anchor.title != "") ? anchor.title+" (Este link se abrirá en una nueva ventana)" : "Este link se abrirá en una nueva ventana";
anchor.className = (anchor.className != '') ? anchor.className+' external' : 'external';
}
}
}
)