Implement export interface class

This class defines the needed interfaces for the kiwi and autoyast
exporter.
Write exports in a subdirectory

Write exports to a subdirectory based on the name and the export type.
This fixes the potential issue that --force deletes non machinery folder
content.
Adapt documentation to new behavior
Merge pull request #472 from SUSE/write_exports_into_subdirectorys

Write exports into subdirectorys