Extension module of golang logging

This is a re-upload of https://github.com/qiniu/log ; which is missing for no clear reason. This module is depended on by qiniu/logkit ; which is Apache 2.0 license. That is why the Apache 2.0 license has been added here. This was originally released to open souce in 2016 along with logkit.

Should you know what became of qiniu/log, and why it is no longer present on Github, please let me know.