If log.SetReportMethod(true) then method=PACKAGE.FUNCTION will be added
as a field to log lines.
eg: time="2016-11-25T19:04:43-08:00" level=info method=main msg="log
testing"
TODO: documentation, examples
`std.Level` is protected by mutex in setter (SetLevel),
so it must be protected in geetter (GetLevel) too.
Signed-off-by: Anton Tiurin <noxiouz@yandex.ru>