Allow producing scale bar and north arrow as separate images

[jahow]

Currently the scale bar and north arrow are blended in the printed map.

Allowing to receive them as separate images would make printing PDF compositions easier, i.e. by putting these elements outside of the map, while still being assured that the scale bar is correct.

This would require also giving the size of the separate images in real world units if possible. Otherwise it would be tricky to include the scale bar as a separate element in the PDF.

Another option is to add getScaleBar and getNorthArrow APIs which could generate the images from a spec, without waiting for the whole map to be generated.

[jahow]

Done in #46