Add expires tag to sitemap for expired content

[hayesgm]

Added support for tag as described by Google: http://support.google.com/customsearch/bin/answer.py?hl=en&answer=2631051.

[kjvarga]

Thanks. I can merge this once I add a test.

[hayesgm]

@kjvarga Thanks-- sorry that I issued the pull request without adding a test first.

[kjvarga]

Thanks for the code! I've included this in 4.3.0. Unfortunately the expires tag hasn't been added to a sitemaps schema file yet, so sitemaps using this tag technically don't validate.