Remove Player.cancel()

See https://github.com/web-animations/web-animations-js/issues/136
Remove superfluous and unimplemented TimedItem.seek()
Remove superfluous TimedItem.reverse()

This functionality will probably be moved to Player.reverse(). See
https://github.com/web-animations/web-animations-js/issues/137
Fix AnimationGroup.clear()

Also remove superfluous TimedItem.clear()
Merge pull request #138 from steveblock/specUpdates

A few bug fixes and updates from the last meeting