Renamed libjass.tags to libjass.parts
Miscellaneous tiny things.

- Moved Webkit-specific keyframes declaration before generic declaration.
- Use fully-qualified type names in JSDoc
- Renamed *Karaoke.value to *Karaoke.duration
- Whitespace
Clear animation styles when clearing pre-rendered subs.
Implemented \fade

- More fiddling with AnimationCollection.