def get_request_token(params = {})
      params[:oauth_callback] = params.delete(:callback_url)
      response = client.get_request_token(params)
      RequestToken.new(response[:oauth_token], response[:oauth_token_secret])
    end
def get_request_token(params = {})
      params[:oauth_callback] = params.delete(:callback_url)
      response = client.get_request_token(params)
      RequestToken.new(response[:oauth_token], response[:oauth_token_secret])
    end
@param params [Hash]
 @option params [String] :callback_url If the user is being authorized
   through another web application, this parameter can be used to redirect
   the user back to that application.

 @return [Flickr::OAuth::RequestToken]