New building and enemy type

Additions:

• Added shield generator building, which creates a shield that reflects projectiles
• Added evasive UFO enemy type, which is capable of avoiding asteroids

Changes:

• Changed font sizes to reduce UI clutter
• Changed asteroid and UFO spawn positions to reduce collisions
• Changed UFO shooting interval and increased minimum target altitude (for balance)
• Added a guaranteed gold asteroid every wave, and reduced credit reward from $15 to $10
• Extra tip in the about page ;)