Fix style in test-circle-square.html
Fix a bug in TimedItem._iterationTimeToItemTime()

When the current iteration direction is backwards, invert the iteration time
delta. This has the effect of using a start time relative to the end of the
iteration interval for reversed iterations.
Use stable sort when calculating the compositing order

This fixes https://github.com/web-animations/web-animations-js/issues/81
Merge pull request #92 from steveblock/sandyBug

Fixes https://github.com/web-animations/web-animations-js/issues/81