Start game when a tile is clicked.

If game is over, it can be started by clicking on a tile.
Merged start-game end-game and level-up buttons.

Since clicking a tile starts the game,
  start-game button is not useful anymore.
  Replace start-game with reset-game.
Put start-game end-game and level-up buttons on top of each other.
  Make only one of them visible at a time.
Scaled images to fill the tiles.

.tile img { height: 100%; width: 100%; }