Adding "callback" capabilities to a response

Added the capability to send asynchronous request in addition to the initial response.  Here is a YAML representation.

-  request:
      method: [POST, PUT]
      url: /rest/callmeback.*
      headers:
          x-reference: "(.*)"
          x-return-url: "(.*)"
      
   response:                     
      -  status: 202
         headers:
            content-type: text/plain
         callback:
                method: POST                
                url: <% headers.x-return-url.0 %>
                latency: 10
                headers:
                    content-type: application/json
                body: >
                    {
                    "id":"20140222144501-308",
                    "reference":"<% headers.x-reference.0 %>"
                    }
                    
      -  status: 429
         headers:
            content-type: text/plain                           


I tried re-using as much as possible .. hope you like it :D  this is my first pull request ever !!

Thanks for the nice stubby btw :)

/Antoine