Will/use mixed modulestore in tests

teardown the content database after tests

Payment/Shopping Cart Backend

and add the ability to record the fulfillment time

* 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!

when in a dev environment.