Merge pull request #1765 from dingpinglv/Iss4757_ReleaseAlpha2

Issue #4757: add an assert to cc.Sprite for debug friendly.
Issue #4757: update the version number of Cocos2d-html5 v3.0 alpha2
Merge branch 'develop-3.0' of https://github.com/cocos2d/cocos2d-html5 into Iss4757_ReleaseAlpha2
Issue #4757: update the build.xml of Jsdoc_toolkit