Added foundation for documents
Added Document type agent model and interface
Implemented standard document class
Added default invoice template
The logo is now set in the document table but the document has to be configured
Replaced old print operations by new routine
Added missing translations
Renamed document template fields
Made template fields mandatory
Renamed iso_document_invoice to iso_document_default
Adjusted document class to new template choice
Added a separate document title
Added title to pdf
Added billing address to default document