Add Continuous Integration and LuaCheck (#71)

All warnings have been fixed as well.

skinlist.lua

@@ -1,7 +1,7 @@
 local skins_dir_list = minetest.get_dir_list(skins.modpath.."/textures")
 
 for _, fn in pairs(skins_dir_list) do
-	local name, sort_id, assignment, is_preview, playername
+	local name, sort_id, is_preview, playername
 	local nameparts = string.gsub(fn, "[.]", "_"):split("_")
 
 	-- check allowed prefix and file extension