Finalize ver0 tags schema

[Jason Gerard DeRose]

lp:749699 basically wrapped up the changes needed for the version 0 schema, but didn't quite finish the tags schema, which is important enough to handle in a separate bug. Don't have time at the moment to do a big write up, but will basically look like this:

tags: {
  'cats': None,
  'dog': {
    'start': {'frame': 123},
    'end': {'frame': 789},
  },
}

Meaning the entire video clip is about 'cats' but there is a 'dog' in video frame 123 to 789 (inclusive). The tagging is very flexible, more flexible than I originally envisioned. But after talking with a number of pro video editors, I came up with this new improved design.