[account] Relation between documents should be explicit many2many with controls

Context.
Actually, when you make an in_refound or an out_refound, you need to put explicit description between the origin invoice to take a control for the reason why this documents where done.

Problem.
In companies with medium quantity of transaction only a description or note is not enough, because the audit tasks become in a really big work.

Solution.

With a simple:

    _columns = {
        'parent_id':fields.many2one('account.invoice', 'Parent Invoice', select=True),
        'child_ids':fields.one2many('account.invoice', 'parent_id', 'Debit and Credit Notes'),
    }

And some operative validations on views, it is posible, we think it should be a big impovement!.

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

We made this, it should be important to be reviewed, we will put this in production at march-01 if somebody have some question and note some needed These are welcome but we think that this behaviour "if exist" should be in data base level.