Fix: Don't crash on kick without user

If you do "/KICK " and don't specify a user the server crashes

home/ben/workspace/ircd.js/lib/commands.js:123
        userNames = users.split(','),
                          ^
TypeError: Cannot call method 'split' of undefined
    at Object.Commands.KICK (/home/ben/workspace/ircd.js/lib/commands.js:123:27)
    at Object.Server.respondToMessage (/home/ben/workspace/ircd.js/lib/server.js:171:36)
    at Object.Server.respond (/home/ben/workspace/ircd.js/lib/server.js:181:14)
    at Object.Server.data (/home/ben/workspace/ircd.js/lib/server.js:320:10)
    at Carrier.<anonymous> (/home/ben/workspace/ircd.js/lib/server.js:277:51)
    at Carrier.emit (events.js:95:17)
    at /home/ben/workspace/ircd.js/node_modules/carrier/lib/carrier.js:71:15
    at process._tickCallback (node.js:419:13)


Doesn't happen anymore with this fix and just sends an "needs more arguments"