function openExternal(){
    if (!document.getElementsByTagName) return;
    var anchors = document.getElementsByTagName("a");
    for (var i = 0; i < anchors.length; i++){
        var thisAnchor = anchors[i];
        if (thisAnchor.getAttribute("href") && thisAnchor.getAttribute("rel") == "external"){
            thisAnchor.target = "_blank";
            thisAnchor.title = "Abre en una nueva p\u00E1gina";
        }
    }
}
document.observe("dom:loaded", openExternal);