Bump debian directory

This commit includes:
* bump debhelper and standards version to current,
* bump Python dependency to 2.7 (it's the default Python version
  in Debian stable; only ever going up from here),
* machine-readable copyright (DEP5),
* replaced rules script with simpler *.install files,
* renamed etc/init.d/live-installer -> debian/live-installer.init,
* removing postinst/postrm (all that was present before is already
  handled automatically with dh_* or dpkg triggers).