Add support for AWS.HttpResponse.createUnbufferedStream()

Allow users to implement low-level streaming without `createReadStream()` when low-level access to headers is required. Note that the `createUnbufferedStream()` method must be called before `httpData` (`httpHeaders` at the latest).

```js
s3.getObject(params).on('httpHeaders', function(statusCode, headers) {
  if (statusCode < 300) {
    if (headers.etag === 'xyz') {
      // pipe the stream, disabling buffered reading
      this.response.httpResponse.createUnbufferedStream().pipe(process.stdout); 
    } else { // abort this request and set a better error message
      this.abort();
      this.response.error = new Error('Invalid ETag');
    }
  }
}).send(console.log);
```