Fix upload path issue with easy thumbnails backend and corresponding documentation update

Thanks for the previous update to fix the easy thumbnail backend issues.  After testing, I found that the returned path omits (depending on what you provide) the parameter provided to 'UPLOAD_DIR' or the default upload path which is 'uploads.'

This proposed commit fixes both these issues by returning the correct path.  I've also updated the docs with a usage example for the easy thumbnails backend.