This fixes cases where a normal user tries to execute the program and
the program raises pbs.CommandNotFound exceptions because it tries to
import a command from sbin, which is not in the path.

Hooray! This is the first release that actually works!

This is done checking an Ubuntu image for the presence of
kubuntu-desktop package.

* Add foreach_file function in OSBase. This is used by all cleanup
  methods
* Add slackware class. This class derives from linux and overwrites the
  log cleanup behaviour. This is needed since slackware stores all
  package management info in /var/log/packages
* Add a shrink method in DiskDevice. This method shrinks the last
  filesystem to the minimum size
* Make the code style comply with pep8

Make the cow file sparse
Add method in Device to list the partitions
Add a "is bootable" flag in Device constructor