Debug draw

[Boscop]

I implemented the debug draw feature, added doc comments for how to use it and an example.
Users have to impl the Draw trait and create a new DebugDraw instance with it.
Then they register it with world.set_debug_draw():

	let mut debug_draw = DebugDraw::new(MyDebugDraw::new());
	debug_draw.set_flags(!0); // set all flags, draw everything
	world.set_debug_draw(&mut debug_draw);
	world.draw_debug_data();