Merge pull request #525 from dstockwell/rename-keyframe-effect

Rename KeyframeAnimationEffect to KeyframeEffect
Merge pull request #527 from dstockwell/remove-active-duration

Remove ability to override activeDuration.
Add previousSibling and nextSibling
Merge pull request #529 from Jackyjjc/master

Add previousSibling and nextSibling
Rename PathAnimationEffect to MotionPathEffect.

Spec change: https://github.com/web-animations/web-animations-spec/commit/7dd177949f49ccdf8593c8178a3a1710910bc44d