Credit notes analysis

[Geert Janssens]

I have spent some time to investigate what is needed to support credit notes 
in GnuCash. The result of my analysis can be found here:
http://wiki.gnucash.org/wiki/Credit_Notes

For simplicity I'm using the term "credit note" as the inverse of all invoice 
types we support so far (Bill/Invoice/Employee Voucher), not just for customer 
related transactions.

My conclusion would be that it should be possible to support credit notes 
without any change to the data model, although maybe one internally used field 
might hold some new values. It will require a minimal set of compatibility 
fixes the the current stable GnuCash version to  avoid some unwanted 
confusion, but those are minimal.

I intend to start with an implementation of this fairly soon.

I'm welcoming all feedback, questions, remarks,...

Geert