3.5.0 released, 4.0.0 ideas

[halfbyte]

This is a smaller release just incorporating the latest PR from @SubandiK.

The road to 4.0.0

This could very well be the last 3.x release, as I think I want to somewhat overhaul the whole thing for a 4.0.0 release. Back when I wrote the initial version to just scratch my own itch, I tried to go the easiest route possible, hence the heavy usage of Struct for example. Here's the ideas I have:

• I think we can deprecate or rather directly remove ZUGFeRD 1.0 - I don't think anyone at this point should still want this.
• I really want to be able to support different Factur-X profiles. There's a couple of things I want to do to accommodate for this:
  • Separate out the XML serialization from the data model. This should make it easier to test these separately but also would make the classes a bit nicer to read.
  • I think the various profiles would lend themselves to some class hierarchy, as profiles are documented as supersets as well
  • I'm not entirely sure if we need the profile structure in both the data model and the serializer.
• One big bucket list item is corrected invoices and credit notes. These seem to be somewhat complex, but they are important.
• I think we need to tighten our validation game in the sense that we should probably include proper validations for at least a subset of all of the business rules defined in the standard.