Inputs: Coins/bills, drink selection
Outputs:
	Return values: Soda, change
	Side effects: Inventory of machine is decreased, available change is increased.
Constraints: Cannot return more of a given soda than is available in inventory.
	Cannot return more change than is present in machine.
Assumptions: Only small bills and coins in US currency will be inserted.
	Only the money for one can will be inserted at a time.