feature: ImageExporter georeferenceing

[Dirk--]

writing some georeferences to file would be a nice feature:

List of some possible formats: http://wiki.openstreetmap.org/wiki/Exporting_calibrated_maps#Calibration_file_formats

Maybe even .kap, a popular format of nautical charts:
http://opencpn.org/ocpn/kap_format

[elphick]

Great tool, but agree @Dirk-- , I needed to deliver a cropped georeferenced image. I've got a working solution that I'll tidy up. Not much activity here for a while - hopefully a pull request can be approved...