singleton, holds a stack of AutoReleasePool*.
- push(): push a new AutoReleasePool into top.
- pop(): pop a AutoReleasePool, clear the pool. (when stack’s size > 1), called every frame in CCDisplayLinkDirector::mainLoop().
- AutoReleasePool holds an array of CCObject(Ref)*, clear() will release each obj.
PoolManager created by sharedPoolManager(), first called in CCDirector’s init() finalized in CCDirector’s destructor, by calling purgePoolManager()
AutoReleasePool created by PoolManager::push(). deleted by PoolManager::pop().
##main method AutoReleasePool - addObject(): put obj into m_pManagedObjectArray