allow for sane logging behind a reverse proxy

Allow the use of headers for logging that are related
to reverse proxy setups:
* X-Forwarded-For
* X-Forwarded-Host
* X-Forwarded-Proto
* X-Forwarded-Protocol
* X-Forwarded-Server
* X-Forwarded-Ssl
* X-Requested-With
* X-Do-Not-Track
* DNT
* Via

Introducing a new logger format alias 'rproxycombined'
consisting of the X-Forwarded-For header + the COMBINED alias.
Merge pull request #18 from jdsn/reverseproxylog

allow for sane logging behind a reverse proxy