Support different projections in OziExplorer file

[gregrs-uk]

Hi and thanks very much for taking the time to create this useful script.

In Great Britain, Ordnance Survey maps (used by many hikers etc.) use the British National Grid (EPSG:27700). This system is widely used to specify co-ordinates and so it is useful to be able to use this projection and co-ordinate system in Mapnik.

It would be great if Nik4 was able to generate an OziExplorer file which automatically grabs the projection from the Mapnik XML file so that projections such as this could be supported. This currently seems to be hardcoded here.