Update away from files based storage

[bell07]

Thanks for this great mod at the first!

I see the mod is not updated since years, in the meantime the minetest does support mod-meta storage and player-meta storage. Using io.open() no longer really welcome. So this mod needs an update for them.

the player does have get_attribute() and set_attribute(), but I do not recommend it to use for money because only available if the player is logged in. (No payments possible to offline users)

The beter way is trough local mod_storage = minetest.get_mod_storage() and then mod_storage:set_string(money_"..name, value)