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, also during rotation yaw values are available on the server.
Simple use cases would be mobs, e.g. slimes to make rotation animations smoother and more precise (not like automatic rotation).

Acces from lua e.g. using
self.object:rotate_yaw(math.pi, math.pi/2)
for half a rotation in 2 seconds.