Update movietheater.js

-New object: purchaseTicket for the ticket purchasing, along with some discount detection. 
-Groups larger than 20 or larger than 200 can get a special discount. 
-Note: test values in welcomeScreenSelection === 2' to demonstrate output for discounts
-Added employeeDiscount property; will be added soon
-purchaseTickets and movieShowing needs integration with each other...coming soon!