Add animated-overlay.gif on custom theme directories too. Fixes #88

- The image file `animated-overlay.gif` was previously included in the package
  at: `./development-bundle/themes/base/images/animated-overlay.gif`. Now, it's
  also included at:
  . "css/" + theme.folderName() + "/images";
  . "development-bundle/themes/" + theme.folderName() + "/images";
  . "development-bundle/themes/" + theme.folderName() + "/minified/images";
- Testcase updated accordingly;