Implement /world set spawn

This allows Management members to move the world's spawn position.
Implement /world set pvp

This allows members of Mineground's staff to toggle whether PVP should
be allowed in the current world. Lots of machinery to support "default"
still has to be implemented, which allows players to toggle it
individually.
Implement /world set difficulty

This allows staff members to change the difficulty level of the current
Minecraft world.
Implement /world set animals

This determines how many animals can spawn per chunk in a world. NPCs
are very stressful at the CPU, so enforce a maximum value of "50" until
we understand the impact better.
Implement /world set mobs

Like animals, also make it possible to change the amount of mobs which
should spawn. Up the maximum number of entities per chunk to 150, since
70 was the default for an easy world.