Rename KeyframeAnimationEffect to KeyframeEffect

Spec change: https://github.com/web-animations/web-animations-spec/commit/94b026972674cfe14a28da17037d761a8d25b484
Remove ability to override activeDuration.

Spec change: https://github.com/web-animations/web-animations-spec/commit/d00254dd8a79ae5f06b3db144f37302bda4f69ce
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
Merge pull request #526 from dstockwell/rename-path-effect

Rename PathAnimationEffect to MotionPathEffect.
Convert timing property values to numbers
Merge pull request #531 from barmstro/fix-defineProperty

Convert timing property values to numbers
Fix polyfill to match spec's definition of the active phase for a timed item
Avoid checking fractional values in test
Merge pull request #537 from alancutter/textShadowTestRebase

Avoid checking fractional values in text-shadow test
Update Firefox 28 failure expectations for path test
Merge pull request #539 from alancutter/fixFirefoxPathTest

Update Firefox 28 failure expectations for path test
Merge pull request #536 from alancutter/activePhaseFix

Match spec's definition of the active phase for a timed item
Updated CustomAnimationEffects to EffectCallbacks
Rebased tests to use EffectCallbacks instead of CustomAnimationEffects
Fixed indexing array without checking array size issue in _getDistributedKeyframes
Merge pull request #540 from alancutter/effectCallback

Replace CustomAnimationEffects with EffectCallbacks