Save a newly generated thumbnail.

        path:
            path of the source image
        key:
            key of the thumbnail
        format:
            thumbnail's file extension
        data:
            thumbnail's binary data

def save(self, path, key, format, data):
        """Save a newly generated thumbnail.

        path:
            path of the source image
        key:
            key of the thumbnail
        format:
            thumbnail's file extension
        data:
            thumbnail's binary data
        """
        thumbpath = self.get_thumbpath(path, key, format)
        fullpath = os.path.join(self.out_path, thumbpath)
        self.save_thumb(fullpath, data)
        url = self.get_url(thumbpath)
        thumb = Thumb(url, key, fullpath)
        return thumb