bake: Tweaks to the `sitemap` processor. Add tests. * Now using a `PageIterator` to get the expected behaviour from a given page source, such as skipping taxonomy pages and such. * Fix formatting of priority. * Use the bake time for `lastmod` attribute.

[pytest]
norecursedirs = .* _darcs CVS .svn .git .hg *.egg-info dist build node_modules venv tmp* {args}