Move all overall game logic to GameManager object

[wesleychchan]

For object specific logic, keep it in the corresponding game object

The GameManager object should:

• Determine when an order should be generated (and pass on the call to the ticket object)
• Determine when the pizza should move into the screen
• Ensure correct starting and stopping of the game
• When pizza reaches middle of screen, awaken toppings. When pizza moves off screen, sleep toppings