fixes

item name is no longer a pk, para pwedeng two same items pero
diff price (for diff customers) in the db
and since pwede same item pero diff drid (as in the case of a
returned item). bale pagkagamit ng addReturnedItem(),
pakigamit na rin ng deleteItem() para sa item na may old drid

add item search function viewItem() for single item search

added DateSearchTrans() for searching for transactions between
two specific dates
added referral to db model in transaction
added referral to transaction bean and addTransaction()
revised erd and class diagram