Phaser.Physics.World.destroy() is never called

This isn't really meant to be merged, just wanted to give you a working example of what's going on.
Right now `destroy()` method of `Phaser.Physics.World` is never called even though `clearWorld` flag is set to **true** by default when calling `state.start()`. 

As a result p2's world isn't cleared and the number of `physics.world.bodies` is increasing every state launch.

Following code will log 'mummy' twice after moving to 'GameOver' state.