Accrual/deferral engine

OpenERP does not have an accruals/deferrals engine (accruals are sometimes referred to as prepayments and deferred incomes).

The implementation should:
- define accruals/deferrals rules
- For deferrals:
-- when registering an invoice, allow to select deferral rules for the invoice lines (for instance, supplier invoice registered in january, but cost relevant for january-february-march)
-- deferrals should be created automatically at the end of a period (menu entry in periodical processing), if needed also with the corresponding reversals for the following period
- For accruals:
-- allow to create accrual models for generating accrual entries
-- generate accrual entries from models automatically at the end of a period (menu entry in periodical processing), if needed also with the corresponding reversals for the following period
-- allow to create manually accrual entries, if needed generating automatically the reversals for the following period
-- when registering an invoice, allow to select accruals related to the invoice