Fixes #356 - More improvements for Debian package specification

• Added rake task pkg:deb to build a Debian package for Foreman with debuild.
• Relaxed dependencies in Debian control file to hopefully allow installation of the foreman DEB under Ubuntu karmic, Ubuntu lucid, Debian lenny and Debian sid.
• Fixed error in preinst script when Puppet is not installed and group 'puppet' doesn't exist yet.