makefile:
- new target "make undist", which deletes the upstream tarball
- $DISTDIR variable for make (un)dist, which defaults to ".." and can be used to create upstream tarballs in arbitrary locations
- refactoring, all function names and global variables now start with "make_" to make the script `source`ing friendly
- made `source`able
added python-dateutil dependency
added python dependency to fix lintian warning