code: function scriptTag(src, callback)  {
  		
  		var elem = doc.createElement('script');		
  		elem.type = 'text/' + (src.type || 'javascript');
  		elem.src = src.src || src;  
  			
  		elem.onreadystatechange = elem.onload = function() {
  			var state = elem.readyState;
  
  			if (!callback.done && (!state || /loaded|complete/.test(state))) {
  				callback.call();
  				callback.done = true;
  			}
  			
  			// cleanup. IE runs into trouble
  			if (!ie) {			
  				head.removeChild(elem);
  			}
  		}; 
  		
  		head.appendChild(elem); 
  	}
docs: if callback == true --> preload