Add Schema.org VideoObject RDFa metadata

* https://en.wikipedia.org/wiki/Schema.org
* http://schema.org/VideoObject (RDFa)

* TODO: duration: ISO 8601 date format (https://en.wikipedia.org/wiki/ISO_8601#Durations)
* TODO: Schema.org/CreativeWork seems to require more structured Copyright ifnormation
* TODO: does <dl> <meta> validate?

I don't have a local installation here, so I'm just editing a template for right now.

Please let me know your thoughts?