When exploring a PDF, it may be convenient to have the typical
informations delivered by some "Document Properties"---and some more
about the MediaBox(es).
So just add the function to get the ModDate and extend the
pdfioinfo example as an example of what the library do have
and pdfioinfo as a debugging tool also.
Signed-off-by: Thierry LARONDE <tlaronde@kergis.com>