Alpha Version 6

Main changes:

• Added a new enemy that fires plasma! Plasma gets larger over time, but the damage it deals reduces as it grows.
• Turrets deployed by the player no longer target the player. Their bullets can still accidentally hurt you though!
• Slightly reduced the difficulty of the game by reducing enemy spawn rate.
• Fixed missiles not being destroyed when they collide with asteroids.
• Increased the score bonus for completing a level.