Support preserveAspectRatio for image elements

Currently in SVGKit preserveAspectRatio is supported for &lt;svg&gt; elements but not for &lt;image&gt; elements. This pull request adds support for supporting preserveAspectRatio for raster images in &lt;image&gt; elements. It even supports the "slice" option which the &lt;svg&gt; support for preserveAspectRatio doesn't support.
Yuck. So the spec has "readonly attribute SVGAnimatedPreserveAspectRatio preserveAspectRatio;" shared between two interfaces that have no base interface. Bad spec :(.

We should externalize this parsing somehow, but I'm not sure how.

Off the top of my head:
1. Create an extension header file imported by the two classes, that guarantees to have this attribute
2. use your code here
3. delete the identical code in SVGSVGElement's postProcessAttributesAddingErrorsTo: method
4. replace it with a call to the method you've got here.

(I'm assuming you copy/pasted the parsing code from there originally, so it's identical?)