add receiving-purchase (except recalculating stock)

new file on
db/patch/
- 20130912_receiving.sql

web/application/helpers/
- receiving.php

alter file on
web/application/controllers/
- retail.php

web/application/helpers/
- purchase_engine.php
- receiving_engine.php
- retail_ajax.php

web/application/views/retail
- purchase/html.php
- purchase/js.php
- retail/html.php
- retail/js.php