[WIP] Selector methods in Response. See #554.

TODO:

* [x] persuade everyone `response.xpath()`, `response.css()` ~~and `response.re()`~~ are cute;
* [ ] add docs;
* [ ] fix tutorial?

/cc @dangra, @pablohoffman, @redapple, @nramirezuy, @shane42, @nyov and everybody else who is interested in API changes.

See #554.