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
Update Player to new timing model and API
Add tests for new player API
Update existing tests to new player API
Update existing test to new player timing behaviour (currentTime does not progress beyond end)
Update test-player-cleanup.html to match new player timing behaviour