Added access of response object to `DataAdapter`

Added access of response object to `DataAdapter` so that extra info such as headers
or cookies can be set on the response by the `DataAdapter`.

Proxying cookies to/from the browser to the API endpoint.

```javascript
DataAdapter.prototype.request = function(req, api, options, callback, res) {
  ...
  // add request's cookies to api call
  cookies = request.jar();
  _.each( req.headers.cookie.split( ';' ), function( cookie ) {
    cookies.add( request.cookie( cookie ) );
  });
  _.extend( api, { jar: cookies });

  request(api, function(err, response, body) {
    ...
    // append the api' response headers to the response of this call
    if ( res && response.headers['set-cookie'] ) {
      res.setHeader('Set-Cookie', response.headers['set-cookie'] );
    }
    callback(err, response, body);
  });
})
```
Merge pull request #2 from saiwong/api_access_to_response_object

Added access of response object to `DataAdapter`