[objc] do something when app ApplicationWillResignActive

http://books.google.co.jp/books?id=_n9WXhy_xjAC&pg=PA151&lpg=PA151&dq=objc+do+something+when+enter+background&source=bl&ots=ArDjYSa1qd&sig=KNYGWzmEwhuJde2Q4gtFSPoUfz8&hl=en&sa=X&ei=LjkMUpnkG4zOkwWVtIE4&ved=0CHcQ6AEwCDgK#v=onepage&q=objc%20do%20something%20when%20enter%20background&f=false