// From http://stackoverflow.com/a/105074
function guid() {
    function s4() {
        return Math.floor((1 + Math.random()) * 0x10000).toString(16).substring(1);
    }

    return s4() + s4()
      + '-' + s4()
      + '-' + s4()
      + '-' + s4()
      + '-' + s4() + s4() + s4();
}