Cell2D 1.5.1 released

Cell2D version 1.5.1 has been released. Aside from the new MobileObject class, this version is mostly just a clean-up of version 1.5.0. The SpaceState rendering weirdness that occurs when your character is on a moving platform or something is finally gone, though, and anyone who wants to know how Cell2D works should have less trouble figuring out SpaceState.move() now that it has actual inline comments.