Added a way to remove an arena without removing all arenas (ticket #4)

Use /spec arena remove <name>

The players inside this deleted arena are desaffected from the arena (arenaNum set to 0).

The last arena is “moved” to the ID of the deleted one, as well as the current spectators inside.

I also changed the way the presence of a lobby is determined to avoid a distance calculation (improves performances).

~~TODO (next commit, don't merge now): don't TP spectators to the lobby if their arena is changed because of another one removed.~~ Done.
> Sorry for the delay, wasn't on yesterday...

No problem :) .

> It would be pretty important for the player to be sent to the global lobby

With `SpectatorPlus.spawnPlayer(Player player)`? Hmm, good idea.  
(By the way this method should be renamed IMHO. But, after the merge to avoid conflicts.)

> with a message saying why: e.g. prefix+"You were removed from your current arena".

I'll add this.

> Spectate mode should stay enabled.

The spectator state is untouched currently.