Output schema.org Microdata (Semantic Web)

The Semantic Web is progressing. The big search engines support the [schema.org](http://schema.org/) vocabulary. HTML5 introduces [Microdata](http://dev.w3.org/html5/md/) for adding semantics.

I've add Microdata output for schema.org terms.
BTW: The fix for issue #461 is also included.

Google refreshes its search engine towards semantics ([WSJ](http://online.wsj.com/article_email/SB10001424052702304459804577281842851136290-lMyQjAxMTAyMDEwNDExNDQyWj.html). An immediate side effect might be better SEO for Octopress blogs.

What do you think about this changes? Is there any interest?