non-buildable (some classes, it relies on, are not yet created). Created interfaces and stubs for BillingCalculator, BillingRules, DefaultBillingRules. Project modified so classes can be included to AcademyBillingTests.
non-buildable. BillingCalculator now will be able to process calls one by one. Separate SubscriberStorage.
non-buildable. Added namespaces.
non-buildable. Method implementations wrapped with namespaces.
Merge pull request #3 from swat32exe/add-classes-with-stubs

BillingCalculator, BillingRules, DefaultBillingRules interfaces and stubs.