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.
Yes, sorry - my bad, when I wrote that I hadn't noticed you'd moved it.

We have a big TODO item of "remove all parsing code from the SVG* classes" (because the spec doesn't allow it to be there). It's all got to move ... somewhere.

FYI: We've been having some good successes with making new "SVGKParser.....h" files for each tag / set of tags. If you look in the "Parsers/Parser Extensions" folder there's a few that people have added. Each one declares an array of tags it "takes owneship of parsing" for, and SVGKParser automatically picks them as needed.

...but that won't work here, because SVGSVG and SVGImage are completely different. So, yes, I think SVGHelperUtitliies is an OK place for now.