create sitemaps model

[MothOnMars]

We need a standard ActiveRecord model for a new Sitemap class. A sitemap should:

• belong to :searchgov_domain (which can have many sitemaps)
• have a non-null, varchar(2000) url database column
  • the url should be unique
  • the url should be read-only
• have a nullable varchar(255) last_crawl_status column
• have a nullable datetime last_crawled_at column
• determine the searchgov_domain it belongs to at the time of creation (see SearchgovUrl#set_searchgov_domain)
• mix in the Fetchable module, which includes additional validations

The spec should include the line it_should_behave_like 'a record with a fetchable url' to include the default Fetchable tests. This class has a lot in common with the other Fetchable classes: IndexedDocument and SearchgovUrl. Those classes and their specs may be helpful. Try to use Shoulda's simple one-line matchers for standard tests such as validations, etc.

[MothOnMars]

@ethanewing , here's the next story for you. Please let me know if you have questions.

[ethanewing]

Will do!

[MothOnMars]

Resolved by #92