v0.19.1225

• Breaking: AddDefaults now also adds the job type definition to the
  IJobsDefinitonService
• New: Implemented a basic specification pattern by providing
  ISpecification<T>, an easy-to-use Specificaion<T> and a set of extension
  methods. Look at the EventFlow specification tests to get started
• Fixed: IEventDefinitionService, ICommandDefinitonService and
  IJobsDefinitonService now longer throw an exception if an existing
  event is loaded, i.e., multiple calls to AddEvents(...), AddCommand(...)
  and AddJobs(...) no longer throws an exception
• Fixed: DomainError.With(...) no longer executes string.format if only
  one argument is parsed