add preloadScene (close fireball-x/dev#477)
add preloadScene (close fireball-x/dev#477)

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