TODO:
- Security:
	- might be an issue if admin changes shop ID while player is using
- Functionality:
	- optimize how refunds are given (e.g. if there is no room for 50s, but room for 1s, then give out 1s instead)
- Aestethics:
	- make colorable with unifieddyes
	- fix stepping sound for "large" node
- Misc.:
	- make usable with "folks" mod
	- set shop name from formspec instead of registration so shops with different names can use same content
	- add player shops
	- fix so unknown items don't mess up shop (may become a problem when shops are registered live)
	- allow shops to be configured live
	- register chat command "/server_shop" to register live
		- "/server_shop add <buyer>|<seller> <id> <item> <price>"
		- "/server_shop remove <buyer>|<seller> <id> <item>
	- use a spinner to select custom amount (example in terraform: https://github.com/x2048/terraform/blob/4b36d36/init.lua#L316 )
	- support groups in buyer shops