Add Continuous Integration and LuaCheck (#71)

All warnings have been fixed as well.

.luacheckrc

@@ -0,0 +1,13 @@
+unused_args = false
+allow_defined_top = true
+max_line_length = 999
+
+globals = {
+    "minetest", "unified_inventory", "core",
+    "player_api", "clothing", "armor", "sfinv",
+}
+
+read_globals = {
+    string = {fields = {"split", "trim"}},
+    table = {fields = {"copy", "getn"}},
+}