Initial Implementation for Issue #565

Here is what I have done.

* Added two tables for customer — *item discount* table and *item group discount* table.

![selection_013](https://f.cloud.github.com/assets/420541/2084758/f0dea042-8e2f-11e3-8d6f-d2f98b2dd1de.png)

* When the user makes a sales order for item `X` with a price list `P`,  it will first search in the customer's *item discount table* whether there is any discount for price list `P`. If found, it will use the specified discount.

* If not, it will then search in the customer's *item group discount table* whether there is any discount for price list `P`. If found, it will use the specified discount.

* If there is no discount specified for item `X` with price list `P`, the discount rate will be zero.

Please, note that the current implementation will apply the discount only if the sales order's price list matches with the price list specified in discount tables.