Update 0.2

This update contains the following:

• Added a new gamemode called Endless - survive for as long as you can against never-ending waves of alien invaders and asteroids.
• Added 3 new levels
• Enemy fire rate is now decreased by 10% instead of 15% on Easy
• Slightly nerfed Alien Interceptor's fire rate
• Slightly nerfed Flying Saucer's fire rate