Implementation for ENHancement 430175: PDF imposition

A single library file pdfimpos.ps is attached to bug #430175 with several
working examples tested in GPL Ghostscript 8.15.  Examples range from a
simple folio of a very plain, well-mannered pdf found in the wild, to a
quarto and sixmo of a more elaborate pdf also from the wild.

The examples demonstrate that the possibilities in designing an imposition
need an API that goes beyond a handful of command line options (and pdfimpose
does), but still it would be natural to have a simple invocation script that
supplied the right specs for a default folio for 2-up duplex on A4 or US letter.

More information in the comments on the bug tracker.

Cheers,
-Chap