[account] Improvement 2 type of document News in_debit and out_debit

Context:

Now we have on account module th option to make 4 basic types of documents:
in_invoice
out_invoice
in_refound
out_refound

With this four document we can (in a basic explanation):

1.- receive money for a client related to an or several Sales Orders. +
2.- refound money to a client related to an in_invoice -
3.- pay money to a Supplier. -
4.- refound money from a Supplier. +

Problem:
Exist a type of document in several countries (We investigated and at least 30 countries around the world use it!) called Debit Note, this document don't have special mathematical behaviour it is exactly in accounting point of view like an in_invoice or out_invoice, but, What is the diference ?.
This Documents _extend_ the original agreement related to an invoice, when you have Sales Orders marked around to a Frame Contract or simply a Contract, you can not legally in some countries, just, make a new invoice that extend the total amounts for this invoices, you need to use a "debit note" that extend the total amount, the lines for this Debit Note can be created from a product, service or simply a little explanation not related at all with any product only an amount.

Example of Case:

You have a sale order with a big company.
This Sale Order is related with a purchase order from your customer.
This Sale Order is with a total amount of €100.
ALL FINE.
But. you have an amount showed on your initial agreement related to Shipping of 10€, and your cutomer before you send the packing call to you and They said that them need the products in an Office 1000Km away from the original destination, we can resolve this operational issue in all countries, simply, cancelling the original invoice and making another new one, but in countries with several fiscal controls, and companies like this example with several internal control can not simply make this, because, you initial agreement not reflect this little amount related with them purchase order. What is the solution?, an "extension" of the initial agreement and related with the last invoice called "debit note".
Other cases are not related with shipping.
- money-market rate due to extra-delay in payment.
- money-market rate due to "extra administrative task"
- extra products due to error on initial agreements.
- all amount related to banks services are called in this countries Debit Notes! because not exist a real invoice to show this amounts.
.. etc.

Our Improvement to proof the concept.

This concept, we could improve it "with internal logic" to not brake the structure of original documents in OpenERP on our localization on trunk-experimental branch, but we think this is enough generical to exist directly on the core of accounting in OErp.:

lp:~javieredm/openerp-venezuela-localization/debit_credit_note