Slot Machine Version 0.4

- Added Sounds for slot machine for the following button events
- Spin
- Bets
- Reset
- Hit Jackpot
- Out of credits
- Click Invalid Spin button, mentioned below
- New button "Invalid Spin" replaces spin button should user, at any
point,
have a bet greater than player's credits
- New button "Quit": simply quits the game
- New background
- Added jackpot win backgrounds, screen flashes to notify player of
winning jackpot
- Added looping background music (Transport Tycoon theme)
- Added a quit button to the top-right corner
- Added placeholder "spinning" reel icon
- Delay after pressing the spin button and after winning jackpot
- Added some comments