prevent multiply scene loading simultaneously (fireball-x/dev#522)
prevent multiply scene loading simultaneously (fireball-x/dev#522)

cocos-creator/deprecated_engine,cocos-creator/deprecated_engine,knoxHuang/engine,fireball-x/deprecated_engine,knoxHuang/engine,fireball-x/deprecated_engine,cocos-creator/deprecated_engine,knoxHuang/engine,fireball-x/deprecated_engine