return lat,lon within a tile given (offsetx,offsety)

def coord(self, offset=(0,0)):
		'''return lat,lon within a tile given (offsetx,offsety)'''
		(tilex, tiley) = self.tile
		(offsetx, offsety) = offset
		world_tiles = 1<<self.zoom
		x = ( tilex + 1.0*offsetx/TILES_WIDTH ) / (world_tiles/2.) - 1
		y = ( tiley + 1.0*offsety/TILES_HEIGHT) / (world_tiles/2.) - 1
		lon = x * 180.0
		y = math.exp(-y*2*math.pi)
		e = (y-1)/(y+1)
		lat = 180.0/math.pi * math.asin(e)
		return (lat, lon)