Redesign: Wrap callback for GET requests
Redesign: Wrap callback for GET requests

- For user code: Use request.callback instead of
  request.parameters.callback to specify JSONP callback function.
- Internal: Use callback_for_jsonp_request instead of request.callback
  to detect whether the response has been returned or not.
- As a result of this change, queued requests are now processed one by
  one regardless of request.method.

kana/fnfnen