Replaces ActiveFedora::Base.url_from_sitemap_index with
ActiveFedora::Base.get_descendent_uris. This was necessary because
the '/sitemap' resource has gone away, and there is no longer a sitemap
to compile a list of urls.

ActiveFedora::Base.get_descendent_uris takes a URI and recursively
looks for descendents, returning an array containing the original
URI parameter and all descendent URIs.

ActiveFedora::Base.get_descendent_uris is used to get a complete list
of URIs to use in ActiveFedora::Base.reindex_everything.

Closes #492.