WIP Implement groups

Working:
Seq groups
Par groups
Pause/play
Reverse (works once)
Set startTime
Set currentTime

I think what we  should do is:

(1) put the Animation setup stuff into a function in this file (setupAnimation?)
(2) put the AnimationSequence / AnimationGroup stuff into a function in a new file (group-constructor.js)
(3) move document.timeline.play into a new file (timeline-play.js) that calls setupAnimation and setupGroup as necessary after checking source instanceof stuffs.