Add a Server interface.

lib/shelf.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library shelf;
 
 export 'src/cascade.dart';
 export 'src/handler.dart';
 export 'src/handlers/logger.dart';
 export 'src/hijack_exception.dart';
 export 'src/middleware.dart';
 export 'src/pipeline.dart';
 export 'src/request.dart';
 export 'src/response.dart';
+export 'src/server.dart';
+export 'src/server_handler.dart';