Dpkg::Source::Patch: New module to generate and apply patches

Dpkg::Source::Patch: New module to generate and apply patches

* scripts/Dpkg/Source/Patch.pm: New module that is able to
generate patches (between files or between directories). It's
also able to apply patches. Built on CompressedFile, it
handles compression/decompression of patches files on the fly.
It still lack some functionalities of dpkg-source (patch
analysis and pre-creation of new directories before patch
application).
* scripts/dpkg-source.pl: Replaced big chunks of the code by
some usage of Dpkg::Source::Patch. More to come later.
* scripts/Makefile.am, scripts/po/POTFILES.in: Register the new
module file.