Invader Movement
	- create InvaderMovementDirection states
	- Initialise their movement
	- in update() call moveInvadersForUpdate()
	- in moveInvadersForUpdate():
		- Make sure 1 second has elapsed before the invader can move
		- only loop over invaders and skip ship and HUD
			- Move the invader 10 pixels to the left right or down

		- set the timeOfLastMove to currentTime so invaders will wait one second to move