Grunt: update generate_themes task to use the new DownloadBuilder API
Grunt: update generate_themes task to use the new DownloadBuilder API

- DownloadBuilder accepts the config file as an option. It's not necessary
  to workaround it by changing the DownloadBuilder config file anymore;
- DownloadBuilder has exposed a method to generate the theme gallery bundle;

GrayYoung/jQuery.UI.Extension,GrayYoung/jQuery.UI.Extension