Pull request to master

You can use fast enumeration for this for loop:
```
for (EnemyViewController *enemyView in self.enemies) {
   [enemyView setIsEnemyPermanentlyExpired:YES];
}
```

Just for sharing, you could also make a custom object just for the enemy array. Other than this object containing the list of enemies, it can have additional behavior such as generating the enemies and also cleaning up after it. So it can do the above code as `[self.enemies expireAllEnemies]`. This is one of the rules in Object Calisthenics (Use First Class Collections). You can google it to learn more :)