Added bash-completion script for our "apt" script.

The script is taken from:
http://forums.linuxmint.com/viewtopic.php?f=148&t=82867

Big thanks to the original author.

Differences from the original script:
- fixed _comp_dpkg_installed_packages and _rpm_installed_packages
  calls for modern systems (thanks to http://askubuntu.com/a/445638);
- made it recognize the "policy" command;
- made it complete the "version" command's argument only for the
  installed packages, because otherwise the output of that command
  would be empty.

Fixes https://github.com/linuxmint/mintsystem/issues/4.