Product Price fields and Last Price in Invoice

closes #445 (closed) and #444 (closed)

New table for storing last price of sale/purchase of a product by a party.
New fields in product table - prodmrp, prodsp to store MRP and Selling Price.
Changes in APIs of product to consider the new fields.
New function to update last price.
Changes in gkupgrade() - added calls to alter product table, create lastprice table and insert last price details of existing users.
Corrected the query for returning invid after Invoice/Cash Memo is saved.(Used Max to return the latest).