Implement ObjectRef:rotate_yaw(radians, speed) to continuously rotate objects

In action: http://www.youtube.com/watch?v=g0vk8DsjIoQ

Continuously rotates a LuaEntity to a yaw with speed.
Simple use cases would be mobs, e.g. slimes to make rotation animations smoother and more precise.
The main advantage of this over automatic rotation is that both server and client make the entity stop when the yaw is reached and that also during rotation yaw values are available on the server

Acces from lua e.g. using
self.object:rotate_yaw(math.pi, math.pi/2)
for half a rotation in 2 seconds.