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
Merge pull request #77 from qiniu/develop

Release v6.1.3
Merge pull request #95 from qiniu/develop

Release v6.1.4
Merge branch 'develop'
Merge remote-tracking branch 'origin/develop'
Merge pull request #105 from qiniu/develop

Release 6.1.5
Merge pull request #118 from qiniu/develop

Release 6.1.6
Merge pull request #123 from qiniu/develop

Release 6.1.7
update version [ci skip]
merge