Add -v for printing what file is being processed

Will help debug crashes (i.e. easier to know what PDF caused the crash).
Move the save logic to the Map class

I think it makes more safe for the Map to know how to dump to the disk.
The main script is now not aware of image manipulation libraries.
Handle PDF parsing exceptions

Yes, we have a few corrupted ones on the set.
Make it possible to run the PDF processing in parallel

Defaults to one process because this must be used with caution. Each
process is going to be very memory hungry.

The change on the Map class is to make it picklable after calling
Dispose(), so it can be serialized to the main process.
Use VRT for georeferencing images and merging

This is a huge performance improvement, because we only work with
metadata in an auxiliary file instead of rescaling and manipulating
big files.