Pull request to master

For lines 108 - 109, an alternative approach would be to create a method in `GameLabelView` that resets the `gameOverLabel`. Then these two lines can be replaced with:

`[self.labelView resetGameOverLabel];`

This improves readability. And also, the constants `LABEL_GAME_OVER` and `GAME_NOT_VISIBLE` constants may also be moved to `GameOverLabel` since it's the only class using it.