Add Schema.org VideoObject RDFa metadata

Schema.org

* https://en.wikipedia.org/wiki/Schema.org
* http://schema.org/VideoObject (RDFa)

TODO:

* [x] duration: ISO 8601 date format (https://en.wikipedia.org/wiki/ISO_8601#Durations)
* [ ] copyright*: Schema.org/CreativeWork seems to require more structured Copyright ifnormation
* [x] validation:  does `<dl> <meta />` validate?
* [x] language codes: is iso839_1 IETF BCP47?
* [x] how to express 'Pycon XYZ'? PublicationEvent > BroadcastEvent?

I don't have a local installation here, so I'm just editing a template for right now.

Please let me know your thoughts?