Use unsafe_unretained instead of weak for iOS earlier than 5.0
Use unsafe_unretained instead of weak for iOS earlier than 5.0

hanhailong/CargoBay,mattt/CargoBay,vvw/CargoBay,ycaihua/CargoBay,songsinh88/CargoBay,dxd214/CargoBay,LiuDeng/CargoBay