Adds weak reference support

Most memory leaks are caused by circular references.  CCWeakReference is created to solve this problem.

Usage:
-------------------------
```
CCObject *obj = new CCObject()
CCWeakReference ref1 = CCWeakRefenrece(obj);  
CCWeakReference ref2;
ref2 = ref1;
ref2.getObject(); // obj
CC_SAFE_RELEASE_NULL(obj);
ref2.getObject(); // NULL
```

Efficiency:
-------------------------
The only overhead is `releaseWeakReferenceFor(this);` called in CCObject::~CCObject(). But the time it takes is really negligible. It runs at O(lgN) time, in which N is the amount of CCObjects been weakly referenced.