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/exit.lua

@@ -0,0 +1,23 @@
+local ExitScene = Scene:extend()
+require 'load.save'
+
+ExitScene.title = "Exit Game"
+
+function ExitScene:new()
+end
+
+function ExitScene:update()
+    love.event.quit()
+end
+
+function ExitScene:render()
+end
+
+function ExitScene:changeOption(rel)
+end
+
+function ExitScene:onKeyPress(e)
+end
+
+return ExitScene
+