Fix some warnings

Author: Luc

examples/RegisterFunction/RegisterFunction.ino

@@ -3,7 +3,9 @@
 LuaWrapper lua;
 
 static int myFunction(lua_State *lua_state) {
+  (void*)lua_state;
   Serial.println("Hi from my C function");
+  return 0;
 }
 
 void setup() {

src/LuaWrapper.cpp

@@ -5,17 +5,20 @@ extern "C" {
     int a = luaL_checkinteger(lua, 1);
     int b = luaL_checkinteger(lua, 2);
     pinMode(a, b);
+    return 0;
   }
 
   static int lua_wrapper_digitalWrite(lua_State *lua) {
     int a = luaL_checkinteger(lua, 1);
     int b = luaL_checkinteger(lua, 2);
     digitalWrite(a, b);
+    return 0;
   }
 
   static int lua_wrapper_delay(lua_State *lua) {
     int a = luaL_checkinteger(lua, 1);
     delay(a);
+    return 0;
   }
 
   static int lua_wrapper_print (lua_State *L) {