Added extra buttons:

- Implemented retry button*
- Escape on mode select sends you to title screen
- Escape on title screen closes the game
- Added "Exit Game" entry on title screen, closes the game when selected

*Pardon my angry comment in `scene/game.lua`.

scene.lua

@@ -7,6 +7,7 @@ function Scene:update() end
 function Scene:render() end
 function Scene:onKeyPress() end
 
+ExitScene = require "scene.exit"
 GameScene = require "scene.game"
 ModeSelectScene = require "scene.mode_select"
 InputConfigScene = require "scene.input_config"