Added different behaviour for zombies and humans.

Zombies now check their surroundings for humans and
chase a random human around. If there's no human nearby,
they just randomly shamble around.

Humans take all the zombies in their surroundings, combine
the direction vectors towards them into one big vector, and
run the opposite direction. This means, humans will run away
from large zombie blobs and dodge and weave around zombies
doing so.

Right now, I'll need to increase the jitter or implement sounds,
so zombies aren't stuck in a grid-like pattern once all humans are
chased to a corner. Furthermore, zombies should stick to a target,
or stick to the closest target.