Add enemy generation and quadrant system

Adds a method to generate enemies. You can specify the speed and where they come in on screen. In order to make it simpler to place enemies without having to worry about pixel positions, I came up with a quadrant system that divides the screen into 8 quadrants, and allows you to place the enemy on the borders of those quadrants just by specifying an integer between 1 and 7. It will automatically place the enemy at this position just above the screen. You can also specify a speed in pixels-per-second to move the enemy at.

METHODS THIS COMMIT ADDS:

-(void)generateEnemy:(CGSize)size atEntryPoint:(NSInteger)entryPoint withSpeed:(CGFloat)speed
	make a new enemy object at the specified quadrant (between 1 and 7) at the specified speed in pixels-per-second.

-(NSInteger)coordinatesForCenterOfQuadrant:(NSInteger)quadrant
	for the given # of quadrants in the scene (defined as an ivar right now and set up during init), figures out the x-axis coordinate of the specified quadrant.
	returns 0 if the quadrant is out of range.

generateTestQuadrantsWithNumberOfQuadrants
	generates test border lines so we can see how the enemies will be placed.