! http: remodel `HttpIp` to `RemoteAddress`, fixes #638

- The `HttpIp` type is replaced with `RemoteAddress` which provides first-class support for "unknown" addresses.
- The `X-Forwarded-For::ips` member is renamed to `addresses`
- The `Remote-Address::ip` member is renamed to `address`
Merge pull request #644 from spray/ticket/638

! http: remodel `HttpIp` to `RemoteAddress`, fixes #638