Alien movement, collisions; black hole tweak

Aliens can follow 5 functional paths. I used rand() % 5 + 1 to determine
the path to take.

Aliens can collide with towers now. It isn't perfect, but aliens will
bounce off the surface of the tower somewhat randomly.

Decreased the black hole push-back amount and decreased the time that
black holes are active (from 40 frames to 30 frames). In my testing I
found that black holes are too effective, having 2 black holes and 1
attacker is enough to win the game