CCMenuItem: crash when removing itself fixed

In CCMenuItem::activate() your target->selector will be called. If you remove the menu item within this method, your CCMenuItem will be released and destroyed while CCMenuItem::activate() method is still running. A fix for that problem is to retain and release the CCMenuItem in its activate() method. This way it will be destroyed earliest when CCMenuItem::activated() has been finished.  

This problem is also documented here: http://www.cocos2d-x.org/boards/6/topics/25625