Add create_video; rename some things

* adds steve.richardapi.create_video
* adds steve.richardpi.MissingRequiredData exception
* changes steve.richardapi.create_category_if_not_exists to
  steve.richardapi.create_category_if_missing because it's less
  awkward
* changes steve.util.verify_json to steve.util.verify_video_data which
  is a more appropriate name
Add v0.2 section to CHANGELOG