issue #5335: update cocos2d-js logo and v3.0 beta link.
issue #5335: updated cocos2d-js documentation url
issue #5335: Updated change log for cocos2d-js v3.0 beta
Merge pull request #416 from linshun/master

issue #5335: Updated change log for cocos2d-js v3.0 beta, and updated cocos2d-js logo