Server admin commands

I'm quite confident that the shutdown and restart command work well.

The "kick" command also works as expected for me, but testing on a single machine is somewhat limited.

*Sidenote:* I had to explicitly test whether the channel was open before closing it. The javadoc says that closing a closed channel is a no-op, but I get a ChannelClosedException when the client disconnects normally and the channel is closed already.

Fixes #1033