fix fireball-x/dev#330: assetlibrary should not cache the callbacks if load type is dontCache
fix fireball-x/dev#330: assetlibrary should not cache the callbacks if load type is dontCache

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