Response header enhancements

[graste]

Currently it's only possible to set response headers (cache-control etc.) for local files as they don't have headers. For source URL requests there's a original_header_whitelist and a default header, that is being added. Those are not configurable at the moment – see https://github.com/berlinonline/converjon/blob/master/lib/server.js#L45-L70

I propose two new settings like original_header_whitelist and additional_response_headers to make converjon a bit more flexible.