‘watch’ file to detect new tags on GitHub and download tarballs.

See https://wiki.debian.org/debian/watch/#GitHub
dpkg option to allow building the source package with a outdated tarball plus a patch.

See http://debian-med.alioth.debian.org/docs/policy.html#debian-source-option
packaging: update Debian/Watch to look for released tarballs
build: add 'debian/watch' to Makefile.am