- create constants for invaders
- find InvaderType by using switch statements
- define the invaders and the grid they're displayed in
- define the name used to identify invaders
- create 2 methods:
1. makeInvaderOfType()
	- uses switch statement to determine the color of the invader
	- allocate a sprite with the color(invaderColor) and size(kInvaderSize)

2. setupInvaders()
	- places newly created sprites in the scene
	- lays out invaders in a grid of rows and columns, Each row contains a single type of invader
		-declare baseOrigin and loop over rows
		- choose a single InvaderType for the current row
		- invader position
		- loop over columns
		- create invader + add it to the scene
		- update invader position