add scheduler for v3.0
Merge pull request #233 from u0u0/master

add scheduler for v3.0
add doc for how to add extension to jsb
fix indent
add xcode
fix pic url
fix pic url
fix pic url
fix pic url
Merge pull request #234 from joshuastray/master

add doc for how to add extension to jsb
Merge branch 'master' of https://github.com/cocos2d/cocos-docs
[Cocos2d-JS]v3.0a2 and v2.2.3 upgrade guide added