Add callback to AWS.Request.send()

  Adds a callback to `AWS.Request.send()` to avoid `on('complete')` events. Instead of writing:

  ```js
  var request = s3.listObjects({Bucket: bucket});
  request.httpRequest.headers['ArbitraryHeader'] = 'value';
  request.on('complete', function (response) {
    console.log(response.data);
  });
  ```

  You can now use the same callback syntax that `listObjects` accepts:

  ```js
  var request = s3.listObjects({Bucket: bucket});
  request.httpRequest.headers['ArbitraryHeader'] = 'value';
  request.send(function (err, data) {
    console.log(data);
  });
  ```