Add accessor to compute sockaddr information of flow

I add accessor to get sockaddr information (e.g. [Ocsigenserver](https://github.com/ocsigen/ocsigenserver/blob/3.0.0-cohttp/src/server/ocsigen_cohttp_server.ml#L59)) and remove blank :) ! I created an another function ([this](http://paste.isomorphis.me/qA4)) in intern of Ocsigenserver, but I think it's could be better in Conduit.