Add model for web pages.

Stores the list of URLs to visit next.
Add crawler to scrape recipes.

Doesn't really crawl right now, just grabs the next page from the
database and tries to parse a recipe from it.
Parse sitemaps to find links to visit.

* Given an robots.txt file, finds a sitemap and returns the
  links found inside.
Recursively download sitemaps.
Record discovered URLs.

After parsing a sitemap, save any of the URLs found in the database.
Extract crawler to separate class.
Validate uniqueness of web page URLs.
Avoid storing duplicate URLs.
Merge pull request #5 from atsheehan/crawl_pages

Crawl pages