[#8] Make get_package_dict public and document

So it is more obvious that it can be overridden by custom harvesters.
Remove silly debug message
[#8] Add transform_to_iso method

This can be overridden by custom harvesters willing to support non ISO
formats (like FGDC). It is called whrn the original_document and
original_format harvest object extras are present. Custom harvesters are
responsible for transforming the original document to ISO.
[#8] Make sure only list and dict extras are dumped as JSON
[#8] Rename harvested document model to ISODocument

GeminiDocument has been kept for backwards incompatibility.