Added Error class to RouteSpecification. Added tests per SzymonPobiega's DDDSample.net

This tests the same kind of unit/integration test as Szymon did at: DDDSample-Vanilla/Tests.Domain/Cargo/RouteSpecificationTests.cs

NOTE: This is a first pass only. Lots of refactor possibilities as there is lots of duplication.

This takes care of Issue #22.