Added clocks and a WebRenderer that's less magical than DefaultRenderer.

- Clocks are now used to trigger play, pause and timeUpdate events for renderers. There is a VideoClock that feeds off a <video> (what NullRenderer used to do) and a ManualClock that allows user script to send events (such as a player not backed by a <video>)
- NullRenderer now takes a Clock and uses its events to determine what dialogues to pre-render and draw.
- WebRenderer is a new renderer that's halfway between NullRenderer and DefaultRenderer. It takes in a <div> to render to, the libjass subs wrapper, but it is not tied to an actual <video> element. It is most of the old DefaultRenderer code except the bits that were specific to a wrapped <video>.
- DefaultRenderer now inherits from WebRenderer. It specializes WebRenderer as a renderer that wraps a <video>, just like it used to before. That is, it sets the clock to a VideoClock backed by a <video>, and makes the libjass subs wrapper wrap the <video>.
Fixed typo in JSDoc.