Simple registration request and response parsing and object representation

This module contains objects representing simple registration requests and responses that can be used with both OpenID relying parties and OpenID providers.

1. The relying party creates a request object and adds it to the AuthRequest object before making the checkid_ request to the OpenID provider:

   auth_request.addExtension(SRegRequest(required=['email']))

2. The OpenID provider extracts the simple registration request from the OpenID request using SRegRequest.fromOpenIDRequest, gets the user’s approval and data, creates a SRegResponse object and adds it to the id_res response:

   sreg_req = SRegRequest.fromOpenIDRequest(checkid_request.message)
   # [ get the user's approval and data, informing the user that
   #   the fields in sreg_response were requested ]
   sreg_resp = SRegResponse.extractResponse(sreg_req, user_data)
   sreg_resp.toMessage(openid_response.fields)

3. The relying party uses SRegResponse.fromSuccessResponse to extract the data from the OpenID response:

   sreg_resp = SRegResponse.fromSuccessResponse(success_response)