Add core commands

[jellz]

(can only be implemented after #1)

The server should include some core commands (that can be overridden by external plugins).

Here's a list of commands to add:

• /difficulty
• /gamemode
• /teleport
• /give
• /kick
• /ban
• /unban
• /whitelist
• /clear (inventory)
• /kill
• /stop
• /time
• /summon
• /setworldspawn
• /weather
• /enchant

Thes commands are inspired by the Minecraft server default commands.

[youngchief-btw]

I think this is a good idea.