cocos2dx android back키 적용

요약. -> 안드로이드 하드웨어키인 back키를 누르면 종료되도록 한다.

1. I added a virtual void keyBackClicked(); in MainMenu.h

2. Then in MainMenu.cpp I added this->setIsKeypadEnabled(true); 

3. in MainMenu::init() and added a method:
void MainMenu::keyBackClicked() {
  CCDirector::sharedDirector()->end();
}

출처 http://www.cocos2d-x.org/boards/6/topics/5041?r=10363#message-10363