Updating helper to match current 2Checkout parameters.

-Remove 'fixed' as it is no longer used.
-Make 'merchant_order_id' the order identifier and make cart_order_id
optional as it is only used with the legacy third party cart parameter
set.
-Rename 'auto_settle' to 'third_party_cart' to identify the parameter
set being used. Pass Through Products parameter sales also automatically
settle now so the 'auto_settle' method name is confusing.
-Add mapping for currency through the new `currency_code` parameter.
Use notification to handle INS messages

notification should be reserved to handle 2checkout INS messages
all INS messages are live (not sent on demo sales)
move previous notification logic to return

return should be used to handle the initial passback from 2Checkout
tests cover demo and live sales
add coverage for the extra helper fields
return_url mapping to x_receipt_link_url parameter
tweaked notifications to handle return params
purchase should not be Completed until fraud_status is 'pass'
handle INS and passbacks as notifications
reflect new behaviour in tests
handle default INS message case, interpret credit card processed as completed sale
Merge pull request #994 from Shopify/2checkout-ins

2Checkout Notifications and Return Process