Fix SVG to PNG conversion when a xlink:href uses a URL

Transform the URL into Data URI in order to avoid ImageMagick and
rsvg-convert limitations. This API should be dropped as soon those external
tools can handle remote URLs (ie.: http, https ...) into SVG definitions.

It was included defensive code and a lot of tests to prevent unexpected
outcomes, like raise ConversionError if the download failures and do
not parse empty content.