Update 0.1.3

This update contains the following:

• Added 3 new levels
• Added a new enemy called Alien Mothership - this ship can also fire lasers from the left and right sides. Only 2 of this enemy can spawn and it appears on level 8 or above.
• Slightly decreased the fire rate of Alien Interceptors and Flying Saucers
• Alien Destroyers can now use maneuvers
• Increased enemy spawn time on all levels
• Rebalanced difficulties
• Added 3 new starfield backgrounds
• Backgrounds are now randomized on all levels
• Changed main menu music