Added a way to remove an arena without removing all arenas.
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.
The players are not teleported to the lobby if their arena is changed because another is removed.
Fixed NPE on teleportation to lobby.
Added doc in /spec arena for remove command.
When a player is desaffected from his arena, he is teleported to the main lobby (if available).
A message is sent to explain the situation.
Merge pull request #8 from AmauryCarrade/ticket4

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