Implement purchase orders

[Gavin]

Each purchase order can only be made out to a single supplier so this could be linked to from inventory items with a 'puchase from this supplier' link. This would then start a purchase order.

On receipt of items they should be easily credited into inventory and a payment approved action is required.

Purchase order numbers are supplied in batches eg P10001 - P20000 an interface and logic is required to handle this.