Merge pull request #14 from qiniu/master

Hotfix v2.4.0: update sdk docs
add crc32 for upload
Merge pull request #15 from shijiayun/develop

add crc32 for upload
docs
Merge pull request #18 from icattlecoder/fixbug/v2docs

按文档规范修改文档
Merge pull request #19 from qiniu/develop

merge develop to master