Weapon improvements, AI tweaks

Added new variables for weapons.
- knockback (mass of the bullets)
- fullauto (whether or not the weapon is full auto)
- damage (damage per bullet)
- noise (range to attract zombies on shoot)

Zombies now have a health variable, which is subtracted from upon each
bullet hit. On reaching 0 or less, their rigidbody's mass is set to
.001, causing ridiculous knockback. I would like to get them to 'slide'
backwards, but right now they kind of teleport. I don't know why.