Development complete

Great news: development is complete! Now work on the packaging for the RGCD release has started.

CHANGELOG

• Added skull baddie.
• Added milestones.
• Fine-tuned gameplay parameters.
• Improved boss and mothership AI.
• Added blinking halt sign.
• Added danger warning for when the shield is really low.
• Redrawn a couple of UFOs.
• Improved asteroids graphics.
• Made various optimizations/improvements/fixes.