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
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
Fixed indexing array without checking array size issue in _getDistributedKeyframes