Merge pull request #26 from tarunbhardwaj/develop

Fixed calling super get_sale_price()
Sale should not attach shipping cost with shipment

Sale should not attach shipping cost with shipment as there can be
multiple shipments and all will have same shipment cost.
Merge pull request #27 from tarunbhardwaj/sale_shipping_cost

Sale should not attach shipping cost with shipment
Add get_gates method
Merge pull request #28 from tarunbhardwaj/get_rates

Add get_rates method
Added method to fetch rates