Returns a url to the sprite image.

def _sprite_url(map):
    """
    Returns a url to the sprite image.
    """
    map = StringValue(map).value
    sprite_map = sprite_maps.get(map)
    if not sprite_map:
        log.error("No sprite map found: %s", map)
    if sprite_map:
        url = '%s%s?_=%s' % (ASSETS_URL, sprite_map['*f*'], sprite_map['*t*'])
        url = "url(%s)" % escape(url)
        return StringValue(url)
    return StringValue(None)