* Removed lumberjack from coremain

As is mentioned in 251, this fix removed lumberjack from coremain.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Remove lumberjack from log middleware

As mentioned in 251, lumberjack is not suitable for applications like CoreDNS
so it is removed from the log middleware.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Update log/README.md as lumberjack has been removed
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Adjust default log output from `ioutil.Discard` to `os.Stdout`
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>