Merge pull request #175 from pandamicro/master

H5: Add upgrade guide for v3.0a
H5: Changelog v3.0a - update document urls
H5 v3.0a: Singleton objects doc added
H5 v3.0a: Singleton objects doc ref added in upgrade guide
Merge branch 'master' of https://github.com/cocos2d/cocos-docs