Allow reducing dimensions of Isometry

I'd like to be able to reduce an Isometry3 to an Isometry2 by removing a dimension.

This is useful for, for example, bridging Amethyst with ncollide2d. Amethyst can be used for 2d work but has no specific 2d mode, so it uses 3d transforms even when rendering in 2d. Being able to turn a 3d Amethyst transform into a 2d Isometry for ncollide is useful