Add enhanced inventory features

[cyril]

Rahul Gupta suggests:

1. Add a special feature in the inventory that allows us to set default discounts for each item for each customer. This will help us save time when we do the customer's billing.

2. Add composite items feature - i.e. Items that use other inventory items as components in pre-defined quantities, and we do a sales of the composite items all the component item's inventories are affected.