Adds weak reference support

adds a new class "CCWeakReference", and some test codes in HelloCpp.

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 weak referenced.