Add events for stock increase/decrease on SaleItem

Emit some events before increasing/decreasing stock on SaleItem. That will be
used on a future commit to synchronize the quantity/quantity_decreased
information with WorkOrderItem.

Also, the Event.connect know supports connecting to a classmethod as a
decorator, making the api much cleaner to write, use and understand.

Change-Id: Ibdd895c13a05149eff0610133496bc21180fb3c7