Added the ability to load SVG images from relative paths.

&lt;image&gt; elements should be able to be loaded from paths that are relative to the original SVG source. This implies that that SVGImageElement needs access to the SVGKSource that the SVG came from. This pull request adds the support, making the source available in SVGSVGElement and using it in SVGImageElement to load the image from the relative path.

Note that this pull request adds the file NSData+NSInputStream which comes from https://github.com/tipbit/tidbits and is MIT licensed. If that's an issue then I can easily recreate the file myself.
Please do something with the NSError parameter.

It is sufficient to DDLogError() it - but we mustn't silently drop errors!