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.
I moved the parsing code to a new method in SVGHelperUtilities named parsePreserveAspectRatioFor:. I agree, copy/pasting the parsing code is unacceptable.  I'm not sure whether that location for the shared parsing code is acceptable; that's your call.