title: RestKit - Map to a simple instance variable
body: <p>I am quite new to RestKit. I know that the Object Mapping of Restkit is very powerful. However, in some case, I just want to map to a simple variable. For example, take a look at the following response:</p>
  
  <pre>
  {
      "response": 400,
      "result": {
          "error_message": "Invalid session token"
      }
  }
  </pre>
  
  <p>I just want to know the value of "response" or "error_message". It's quite wasteful to create 2 classes "response", and "result", since these classes have only few fields.</p>
  
  <p>Any recommendation is welcome.</p>