Add the '/as resell' command to let players sell their region to others

After executing the command the player is still owner and can do
everything like normal. The sign changes to the 'resell' section of the
chosen sign profile. The region flags change to the 'resale' section of
the flag profile (other greeting). When a player buys the region it
transfers the money to the previous owner. New schematic event and
commandevents are available for this action. Previous owner will get a
message if he is online.
A command to stop reselling your region is on the TODO list.