Feature #4977: Improve AssetsManager logic
Feature #4977: Fix issue of test case that progress doesn't exist in callback
Feature #4977: Add CCEventListenerAssetsManager.h to cocos-ext.h
Merge branch 'v3_assets_manager' of git://github.com/cocos2d-html5/cocos2d-x into v3_assets_manager
Merge pull request #35 from pandamicro/v3_assets_manager

Improve AssetsManager logic