Simplify server interface

Compatibility breaking interface changes:

• Simplify the Lwt server signature, so that manual construction of
  a callback is no longer required (#210).
  Code that previous looked like:

   let conn_closed (_,conn_id) () = <...>
   let config = { Server.callback; conn_closed } in

should now be:

   let conn_closed (_,conn_id) = <...>
   let config = Server.make ~callback ~conn_closed () in

• Remove the Cohttp.Base64 module in favour of the external base64
  library (which is now a new dependency).

New features and bug fixes:

• Lwt respond_error now defaults to an internal server error if no
  status code is specified (#212).
• Modernise the opam file using the OPAM 1.2 workflow (#211).
• Flush the response body to the network by default, rather than
  buffering by default. The ?flush optional parameter can still
  be explicitly set to false if flushing is not desired (#205).