Video sitemap

This pull request adds support to video sitemaps (more info [here](http://support.google.com/webmasters/bin/answer.py?hl=en&answer=80472&topic=10079&ctx=topic#2)).

* The feature has been implemented only in `XmlSitemap::RenderEngine#render_string`
* There's room for improvement (validation on the specified options, look [here](http://support.google.com/webmasters/bin/answer.py?hl=en&answer=80472&topic=10079&ctx=topic#2) again)
* `XmlSitemap::Item#initialize` needs some refactoring