Major code revision

* 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