return URL for a tile

def url(self, service):
		'''return URL for a tile'''
		if service not in TILE_SERVICES:
			raise TileException('unknown tile service %s' % service)
		url = string.Template(TILE_SERVICES[service])
		(x,y) = self.tile
		tile_info = TileServiceInfo(x, y, self.zoom)
		return url.substitute(tile_info)