Ability to spin the sky in any axis and defaulting to z

Currently `spin`'s only param is `r` (angle), but it always rotates the x axis. I've added `axis` as a param. 

I've also changed the default axis to z, because usually east/west is represented by x and north/south by y (in 2D, z in 3D), so we should be rotating the z axis to make the sun, moon and stars go from the east to the west.