Remove default GunUpgrade from Ships.
Remove an instanceof.
More debug info, show gun upgrades
Make pickup collisions more sensible.
Only show upgrades when there is at least one.
Use a non-asymptote-y function to decide how many enemies to have per wave
Merge cooldownRate into cooldown.
Drastically reduce wave intermission.
Show cooldownTimer without creating a variable for it
Background stars' speed changes with player's ship's vertical speed
Revert 1c42ed5
Merge remote-tracking branch 'origin/master' into rainbow