title: object has no attribute 'add' on python's protobuf
body: <p>I want to test the nested message on protobuf's API on python.</p>
  
  <p>My proto <code>ndemo.proto</code> file was:</p>
  
  <pre><code>package ndemotest;
  
  message BaseRequest
  {
      required bytes Key = 1;
  }
  
  message ContRequest
  {
      required BaseRequest baseRequest = 1;
      optional string Url = 2;
  }
  </code></pre>
  
  <p>My python <code>ndemo.py</code> code was:</p>
  
  <pre><code>import binascii
  import ndemo_pb2
  
  
  contReq = ndemo_pb2.ContRequest()
  contReq.Url="www.google.com"
  
  base_request = contReq.baseRequest.add()
  base_request.Key="12345"
  
  
  packed_data = contReq.SerializeToString()
  
  print 'sending "%s"' % binascii.hexlify(packed_data) 
  </code></pre>
  
  <p>When I ran my script as <code>python ndemo.py</code>, there was an error</p>
  
  <blockquote>
    <p>Traceback (most recent call last):   File "ndemo.py", line 8, in
    
        base_request = contReq.baseRequest.add() AttributeError: 'BaseRequest' object has no attribute 'add'</p>
  </blockquote>