Add missed docs, use class vars for log conf/context/def.format

Both for syslog and local logging, user can specify custom
name for logging config erb template to be used as well as
custom logging context and default format headers

Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>
Merge pull request #1 from bogdando/logging_fixes

Add missed docs, use class vars for log conf/context/def.format