Add HTTP Forwarding headers.

Fixes #30: 

* Adds `X-Forwarded-For`, `X-Forwarded-Proto`, `X-Forwarded-Host` and `X-Forwarded-Server`.  
* Does not trust the X-Forwarded-For from the original request, overwrites it. Consider making this configurable?
* Does not add support for the draft standard of the `Forward: ` header.