Fix for highsource/ogc-schemas-34

This is a workaround for the PhantomJS namespace declaration bug which was causing highsource/ogc-schemas#34.

This fix uses the Jsonix `namespacePrefixes` feature to declare all the namespace prefixes on the root element. Thus `xmlns:xlink` does not need to be declared in the subelements and consequently the PhantomJS bug never pops out. As a side-effect you get a nicer XML with `sld`, `ogc` and `xlink` prefixes.