Get the stored thumbnail if exists.

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

def get_thumb(self, path, key, format):
        """Get the stored thumbnail if exists.

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