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.
Implemented tariffing rules
minor formating fixes
Magic numbers replaced with constants. Ceiling and isWeekend moved to functions
Some formating fixes.
Blocks implementation modified.
Fix .filters file.

.filters file structure was broken. The line with opening tag for
Utility.cpp was misplaced for some reason.
Merge pull request #1 from 4d6178/add-tariffing-rules

Fix .filters file.