Improved the ship's animation

-Added the flametrails
-Fixed the transparency issue
-Removed Ship.invincible_count and gave its job to Ship.invincible
-Moved the flametrail sprites up slightly
-Moved the color key from the top-left pixel to the bottom-left
Tightened some code

-Added documentation in places
-Moved some structures onto one line
-Removed unnecessary global statements
-Made some exceptions clearer
-Added some aliases to frequently-called code to improve performance a bit
Moved the UFO's responsibility of appearance to itself

-Now the UFO decides when to appear on-screen, not ingame.py
-Added a new group for the UFO (adding it to others caused problems)