Check if null before closing
Make sure Police Box chunks remain loaded so that we can manipulate blocks in them when in a TARDIS world

Hopefully fixes the Ploice Box dupe bug - the code was running fine and
said it set the Police Box blocks to AIR, but because chunk was
unloaded, it really didn't do it...
Give Police Box RE-builder its own class so we don't have to pass around a boolean all the time
Add a command to check which chunks the plugin is stopping from unloading
Changed message to reflect addition of handbrake
Add a Y coordinate to areas and check that the start and end blocks are the same Y level

Areas must be FLAT! Add to documentation...
Don't bother replacing same with same
If it's a TARDIS world don't bother destroying the inner TARDIS as were going to delete the world anyway...