Stun and knockback for player

[snipercup]

Requires #440

Mobs can now attack the player with stun and knockback effects.

I made the following changes:

1. removed melee damage property and instead assigned a damage number to the target attributes
2. split the target attributes into "any of" and "all of" so that every attack can target the specified attribute each time
3. implemented a stun attribute, which will prevent the player from moving while active
4. Added melee attack cooldown to better balance the mob attacks
5. added a knockback distance so mobs can attack using knockback.
6. added the iron stalker mob to two urban maps.