Cleanup: moved relationships into own file.

Moved trading and pricing code to another folder, separate files.
NPC: Added further inventory functionality.
Trading: Added price data structure, which allows for setting prices for items. Also includes some default definitions.

trade/prices.lua

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+-- Price table for items bought/sold by NPC traders by Zorman2000
+-- This table should be globally accessible so that other mods can set
+-- prices as they see fit.
+
+npc.trade.prices = {}
+
+-- Table that contains the prices
+npc.trade.prices.table = {}
+
+-- Default definitions for in-game items
+npc.trade.prices.table["default:apple"]  =        {item = "default:iron_ingot",  count = 1}
+npc.trade.prices.table["default:stone"]  =        {item = "default:wood_planks", count = 1}
+npc.trade.prices.table["default:cobble"] =        {item = "default:iron_ingot",  count = 1}
+npc.trade.prices.table["farming:cotton"] =        {item = "default:iron_ingot",  count = 1}
+npc.trade.prices.table["farming:bread"]  =        {item = "default:gold_ingot",  count = 1}
+npc.trade.prices.table["default:sword_stone"]  =  {item = "default:iron_ingot",  count = 2}
+npc.trade.prices.table["default:pick_stone"]  =   {item = "default:iron_ingot",  count = 1}
+npc.trade.prices.table["default:shovel_stone"]  = {item = "default:iron_ingot",  count = 2}
+npc.trade.prices.table["default:axe_stone"]  =    {item = "default:iron_ingot",  count = 1}
+npc.trade.prices.table["default:hoe_stone"]  =    {item = "default:iron_ingot",  count = 1}
+
+
+-- Functions
+function npc.trade.prices.update(item_name, price)
+	for key,value in pairs(npc.trade.prices.table) do
+    if key == item_name then
+      value = price
+      return
+    end
+  end
+  return nil
+end
+
+function npc.trade.prices.get(item_name)
+  for key,value in pairs(npc.trade.prices.table) do
+    if key == item_name then
+      return {item_name = key, price = value}
+    end
+  end
+  return nil
+end
+
+function npc.trade.prices.add(item_name, price)
+	if npc.trade.prices.get(item_name) == nil then
+		npc.trade.prices.table[item_name] = price
+	else
+		npc.trade.prices.update(item_name, price)
+	end
+end
+
+function npc.trade.prices.remove(item_name)
+  npc.trade.prices.table[item_name] = nil
+end

trade/trade.lua

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+-- NPC trading abilities by Zorman2000
+
+npc.trade = {}
+
+npc.trade.CASUAL = "casual"
+npc.trade.TRADER = "trader"
+npc.trade.NONE = "none"
+
+function npc.trade.get_random_trade_status()
+	local chance = math.random(1,10)
+
+	if chance < 3 then
+		-- Non-trader
+    return npc.trade.NONE
+	elseif 3 <= chance and chance <= 7 then
+    -- Casual trader
+    return npc.trade.CASUAL
+	elseif chance > 7 then
+    -- Trader by profession
+    return npc.trade.TRADER
+	end
+end