Inject extracted config files during the autoyast installation

In order to retrieve the extracted config files the user is asked for
the URL to the system description before the installation starts. The
URL should contain the system description content as it is stored in
$HOME/.machinery/.
Also add changed managed files to the autoyast export
Create a more realistic unmnanaged_files directory in the factory
Add unmanaged_files to the autoyast export
Add SystemDescription#description_path convenience method
Copy system description content into the autoyast export directory

The export directory needs to be made available to autoyast via network
during installation so that the extracted files can be put into the
system.
Add unmanaged files filter to AutoYaST export
Be more thorough when importing config_files and changed_managed_files

The script now doesn't try to download directories anymore (which fails)
and only sets the file attributes when they are available from the
manifest.json.
Use "http://" as the default entry for the system description URL
Adapt autoyast export integration test
Require given_filesystem 0.1.1
Do not try to include files if none were extracted
Get rid of superfluous auxiliary variable
Use File.join() to build the file paths
DRY up extracted files methods
Use extracted files with spaces in the factory system descriptions

That way problems with special characters in the tested code will be
caught more easily.
Encode special chars in the URLs that are used for downloading files
Rename "kiwi_helpers" directory to "export_helpers"

This directory will also contain file for autoyast exports in the
future.
Include a README file in the autoyast export

The README contains information on how the export can be used for
AutoYaST installations.
Fix coding style issues