Removed color.py's dependency on the PIL

-All color blending is now handled by Pygame
-Also changed color initialization from (r, g, b) to hexadecimal strings
-Enemies, blocks, and UFOs look different as a result
-Changed some outlines to grey
Removed config.debug in favor of the built-in __debug__

-__debug__ is true if the Python interpreter is run without -O or -OO (which perform optimizations)
-In that case, why worry about that ourselves?
-Also loaded all files with os.path.join, to improve compatibility
-Changed isinstance(x, Block) in blockgrid.py to x is not None (it's more concise)
Moved around some functionality for Enemy

-Moved all EnemyBullets to their own group in ingame.py
-Added a different difficulty ramp for 2-minute and 5-minute modes
-Made enemybullet.py a little cleaner
-Added a function to enemysquadron.py that updates the Enemy class as a whole
All-around code cleanups

-Things on the order of aliases, moving things to one line, aligning definitions, etc.