Merge pull request #42 from qiniu/develop

Release v6.0.0
Merge pull request #43 from qiniu/develop

Release v6.0.0 (patch)
Merge pull request #51 from qiniu/develop

Release v6.0.1 (patch)
Merge pull request #58 from qiniu/develop

Release v6.1.0
Merge pull request #60 from qiniu/develop

Release v6.1.1
Merge pull request #62 from qiniu/develop

merge develop to master
Merge pull request #66 from qiniu/develop

release v6.1.2
Merge pull request #72 from qiniu/develop

add warning of utf8 keys
Merge pull request #74 from qiniu/develop

add comment of download domain
添加续点上传时key为空情况判断
Merge remote branch 'upstream/master' into develop