Fixed an issue with block.py

-One instance of blocks was not change to blockgrid.blocks
Added a column of light to indicate the target column for blocks

-This'll DEFINITELY improve the gameplay
-Fixed a possible issue with enemysquadron generating no enemies (the odds are small, but it's possible)
-Changed a conditional in balloflight.py
-Added a white line to bg.png to show the bottom of the grid
Improved bullet behaviors

-ShipBullet can now kill enemies as they're lowering
-Added a dummy state to UFO to ease ShipBullet comparisons
-All EnemyBullets disappear when the player is killed
-No new EnemyBullets are spawned until the player respawns