Implement the configmapper module.

This module abstracts the access to the configuration file by providing
getters and setters that are mapped to the respective directory entries
of the configmanager module.
Update test script.
Fix style.
Implement D-Bus interface.

All options of the configuration file are now available via D-Bus for
read and write access.
Update D-Bus configuration.

Now all users belonging to the group rbackupd are allowed to access the
D-Bus interface.
Update configuration file.
Move version information into separate sourceable file.

This has two advantages:
    * we cannot import rbackupd in setup.py (it might not exist at this
      time) so we can now load it dynamically
    * the version information is stored in one place and no longer
      scattered around several files