Adds link normalization and .

* `Crawler::Page#links` returns collection of `url`s linked to by that page.
* `Crawler::Page#assets` returns collection of `url`s for all assets included in that page.
* `Crawler::Processor` no longer responsible for normalizing `href`s.
Extracts `href` concerns into separate class.

* `Crawler::Page` no longer responsible for normalization of `href` values.
* `Crawler::HrefNormalizer` responsible for normalization of `href` values.