Added a command feature which allows anyone with the correct permission to use the /removeshop command.

Players with OP or players given access to itemtradeshops.removeshop permission can now look at a shop sign and run /removeshop, to remove the shop from the 'shop.yml' list, enabling the shop to be destroyed or claimed by any player.
The itemtradeshops.removeshop permission is initially defaulted to players with operator.

This update also includes new permissions for existing commands, such as
/buyitem <amount> <item_id> and /costitem <amount> <item_id>.
The permissions for these commands are set to default, therefore any player should have access to these commands when the plugin initalises. the permissions are:
itemtradeshops.buyitem
itemtradeshops.costitem