Implement ghost- aka. sensor-objects

[qrlpx]

Objects who report overlaps/collisions with other objects, but don't actually affect them.
Bullet3d calls them ghost-objects; for Box2d and Chipmunk2d, they're sensors.

[Connicpu]

This isn't the best, but as a workaround you could keep a separate CollisionWorld from ncollide just for handling those overlap queries.

[sebcrozet]

I've been working on this during the past few weeks. I have a first working demo now (not merged with master yet), so hopefully I'll be able to release a first implementation in a week or so!