Adds `Map` for storage of results.

* `Crawler::Map#add` adds a page to the map.
* `Crawler::Map#static_assets` stores all assets found during the crawl.
* `Crawler::Map#site` is the actual `site map`.  This is a simple hash keyed with `URL`s and with each bucket holding a collection of links and asset references.
Renames `Crawler::Processor` to `Crawler::PageProcessor` as the first name kind of sucked.