addImageAsync memory leak fix

Corresponding bug: http://www.cocos2d-x.org/issues/1087
Corresponding forum thread:
http://www.cocos2d-x.org/boards/6/topics/17031
addImageAsync memory leak fix

Bug: http://www.cocos2d-x.org/issues/1087
Thread: http://www.cocos2d-x.org/boards/6/topics/17031?r=20136#