Added an Arena object, used to store an arena.
Added an ArenaManager, to manage and store the arenas.
Automatic migration of the old arenas to the new storage format.
Changed "lobby.vector" to "lobby.location" in the setup.yml file.
The arenas are saved when the server is stopped.

(Just in case someone, in the plugin or through the API, forgot to save.)
Updated the plugin to use the new storage method.
Updated the API to reflect the arenas changes.
Merge conflicts
Added equals and hashCode methods to the Arena class.

Used in the ArenasManager's HashMap used to store arenas in memory.
Merge pull request #12 from AmauryCarrade/newArenaStorage

New way to manage & store arenas