taken from http://stackoverflow.com/questions/105034/how-to-create-a-guid-uuid-in-javascript

guid.js

function s4() {
  return Math.floor((1 + Math.random()) * 0x10000)
             .toString(16)
             .substring(1);
};

function guid() {
  return s4() + s4() + '-' + s4() + '-' + s4() + '-' +
         s4() + '-' + s4() + s4() + s4();
}