initial commit of shopping cart and cybersource integration
added shopping cart list template, embedded form
add parameterization of cybersource creds
Move shopping cart from session into model/db
add Validation function for cybersource receipt POST
add Order model fields for receipt generation
factor out cybersource processor from cart
Lots more verification of CyberSource reply + receipt generation
about page changes, refactor processor reply handling
Add in new VerifiedCertificate order item
Pull CyberSource values from environment variables
when in a dev environment.
Remove enrollment_cost from course_module
Some cleanup fixes to get verified certs working.
Put shopping cart views behind flags
Start building tests
Major cleanup work on ShoppingCart models

* Make currency a property of the Order (for validation purposes)
* Remove null=True from Char fields
* Use InheritanceManager for subclassing OrderItem
* Change VerifiedCertificate to better handle some new use cases
* Cleaned out old migrations
* Tests!
make PaidCourseRegistration mode aware
Adding migration to store purchased mode in PaidCourseRegistration
add handling of CyberSource non-ACCEPT decisions
change method sig of process_postpay_callback