This diff adds support for the automatic generation of a credit note when the "void invoice" button is pressed.
A credit note is simply an invoice with negative amounts on it, and serves as paper trail that a previously issued invoice was corrected.
Both the original invoice and the credit note end up under the "voided" section in Blesta for now.
I also added support for the accrual accounting method to the "tax liability" report.
If that is selected all invoices are included in the report (as opposed to just the paid ones when using "cash accounting")
The credit notes will have a negative tax amount in the report, as you can deduct the amount from your the next VAT payment to the government.
http://www2.noc-ps.com/dl/blesta-3-1-credit-notes.diff