Implement a scoring system.

[wolf81]

This is a comprehensive feature, would need some time to implement.

Suggestions:

• Use the JSON files to add a score for each creature type.
• Figure out if SKLabelNode is high performance (uses bitmap fonts). Otherwise might need another solution for in-game labels that display scores.
• Show player score in the hud.
• Implement a high-score scene - the high score scene is shown when player has lost or finished all levels.
• Users should be able to enter name in high-score scene.