Improve motion path support

The 'auto-reverse' option for the 'rotate' attribute is now supported.
This enabled auto-rotation, and also adds a 180 degree rotation.

We exercise the 'accumulate' option for the animateMotion element.
This corresponds to the iterationComposite option for Web Animations.
Merge pull request #10 from ewilligers/iteration-composite-motion-path

Improve motion path support