Define SVGPolyfill animation element classes

(For merging into project branch smil-in-js)

SVGPolyfillAnimationElement is used as a base class for the stub classes, such as SVGPolyfillAnimateMotionElement, that are used for the JavaScript Polyfill implementation of SVG animation.

The Polyfill will add methods and a property (targetElement) to the prototype for SVGPolyfillAnimationElement.
http://www.w3.org/TR/SVG/animate.html#InterfaceSVGAnimationElement