d/g-d.post{inst,rm}: adding postinst and postrm script

For automated use of a centralized project directory we will use a
seperated group 'gridvis-desktop' that will be generated or removed by
the postinst/postrm scripts.
d/g-d-w.sh: expanding the starter script

The starting script is expaneded to do some sanity checks after adding
the the implementation of the postinst/postrm scripts.
Imported Upstream version 5.1.0
Merge tag 'upstream/5.1.0' into debian/sid

Upstream version 5.1.0

# gpg: Unterschrift vom Mi 26 Nov 2014 20:48:59 CET mittels RSA-Schlüssel ID 251D1DB0
# gpg: Korrekte Unterschrift von "Carsten Schoenert <c.schoenert@t-online.de>"
README.source: adding a basic description

The creation of the orig.tar.xz has to be done thrue various manually
steps. Adding some basic information how to do this.