Migrates the Map database to the new V3.x format, that uses UUIDs 
instead of player names to designate players (among other improvements).
Migration runs in a separate thread, therefore its progress can only be 
watched from the server console.

The migration will run the following steps :
    - Checking if there are files to migrate. If not, the migration stops.
    - Checking if there are backups from a previous migration.
        If there are, the migration stops.
    - Loading the old map and poster data to memory.
    - Backing up old files and new files, to the backups_pre-v3 and
        backups_post-v3 subdirectories respectively.
        Backup's integrity are chacked using file size and SHA1 checksum.
        If integrity could not be proved, the migration stops.
    - Retreiving the UUIDs of the players from Mojang's servers.
    - Checking if some UUIDs could not be retreived.
        If there are, this means some of your players may have changed names
        before the migration started. The plugin will therefore try to retreive
        them specifying a time, back when usernames could not be changed.
        If some names could still not be matched to their UUIDs, then these are 
        probably non-paid accounts.
        If no UUID has been retreived at all, the migration stops.
    §c--- From this step, changes to disk will be made, and you will have to use§r
    §c--- backups if you want to revert back from before the migration started.§r
    - Merging the old map data with the new one, if there is any 
        (which can be the case if your player started to use newer
        versions of ImageOnMap before the migration started).
    - Saving all this merged map data to disk.
    - Removing the old map data from their former files, leaving only the data
        that could not be migrated due to usernames that could not be matched to
        their UUIDs. 
        Original data is still present in the appropriate backup directory,
        just in case.

Note that this plugin will NEVER delete nor overwrite any backup directory.
Moving or deleting these backups is left to the administrator's responsibility.