Implement LISTEN support

[Fox32]

A possible fix for #62.

Usage:

   import 'package:postgresql/postgresql.dart';

   main() async {
     var uri = 'postgres://test:test@localhost:5432/test';
     var connection = await connect(uri);

     final stream = connection.listen('test');

     var sub;
     sub = stream.listen((e) {
       print('$e');

       if (e == 'quit') {
         sub.cancel();
         connection.close();
       }
     });

[adaojunior]

Any plans to merge this ?