public function __toString()
 {
     $e = new Element('link');
     $e->rel('stylesheet')->type('text/css')->href($this->getAssetURL());
     if (count($this->combinedAssetSourceFiles)) {
         $source = '';
         foreach ($this->combinedAssetSourceFiles as $file) {
             $source .= $file . ' ';
         }
         $source = trim($source);
         $e->setAttribute('data-source', $source);
     }
     return (string) $e;
 }