unpack_build_artifacts.sh: added reference to common_jenkins_functions.sh
deb_download_unpack.sh: issue #19; handle multiple packages

- Added new parameter --package-dir, the target of the downloaded
  package(s)
- Added new function set_package_filename_url() to set $PKG_FILENAME
  and $PKG_URL script-wide
- Wrap the downloading and unpacking inside a while[] block
deb_download_unpack.sh: close #20; implement --examples switch