CreditNotes implementation

[phillipp]

We are going to need CreditNotes (Stornorechnung / Gutschrift) for our business case. The implementation itself is not too complicated, but we should make a decision on the naming scheme and if it is going to be breaking anything or not.

I would propose that we use the Xrechnung::Document as a base class and add a new Xrechnung::Invoice and Xrechnung::CreditNote for the fields that are specific for their respective document types. Other document types (Order etc.) could then be added more easily.

This change would naturally break stuff that uses only Xrechnung::Document so I'd say that would be a good time to release a 1.0 that is more or less complete to use in 95% of business environments.

@corny any thoughts?