loadScript.js

function loadScript(src, callback) {
  var script = document.createElement('script');
  script.type = 'text/javascript';
  script.src = src;

  if (callback) {
    script.onreadystatechange = script.onload = function() {
      callback();
    };
  }

  document.body.appendChild(script);
}

// Example Usage
window.onload = function() {
  loadScript('http://feather.aviary.com/js/feather.js', function() {alert('loaded');})
}