Add cocos2d-js v3.0 beta changelog.
added chinese version release note of cocos2d-js v3.0 beta.
Added english version of release note for cocos2d-js v3.0 beta
Updated asset manager document link.
updated update guide for cocos2d-js v3.0beta
Added changed API list
Merge branch 'master' of https://github.com/cocos2d/cocos-docs
updated cocos2d-js readme.md
updated release note
updated chinese version of update guide
updated cocos2d-js release note, changelog and update guide to cocos2d-js v3.0 beta. Added new actions document link.
Merge pull request #307 from linshun/master

Updated cocos2d-js release note, changelog and update guide to cocos2d-js v3.0 beta