Consider exporting physics backends using features

[wackbyte]

You should be able to use a feature named something like "backend-nphysics" to have NPhysicsBackend included directly in the crate (for convenience).

[AndreaCatania]

This is a great idea, but I wonder if it's possible tell cargo to download and link a new crate only when a specific feature is provided.

If you know how to do it, a PR is well accepted.

[wackbyte]

I created #4.