Upgraded Axon Framework to 2.1
Refactoring commands to provide a cleaner event-based API.

Moved ExchangeId and SecurityOrderId from exchange.domain.model to exchange.domain.command.
Refactoring commands to provide a cleaner event-based API.

Added default constructor to ExchangeId that makes use of Axon's IdentifierFactory.
Refactoring commands to provide a cleaner event-based API.

Added default constructor to SecurityOrderId that makes use of Axon's IdentifierFactory.
Refactoring commands to provide a cleaner event-based API.

Introduced AbstractIdentifier to serve as the base class for ExchangeId.
Refactoring commands to provide a cleaner event-based API.

Refactord SecurityOrderId to use AbstractIdentifier.
Refactoring commands to provide a cleaner event-based API.

Renamed SecurityOrderId to OrderId.
Refactoring commands to provide a cleaner event-based API.
* removing dependencies between commands and domain model.
Refactoring commands to provide a cleaner event-based API.
* removing dependencies between commands and domain model.
(working on OrderDescriptor)
Refactoring commands to provide a cleaner event-based API.
* removing dependencies between commands and domain model.
(working on OrderDescriptor)
Refactoring commands to provide a cleaner event-based API.
* removing dependencies between commands and domain model.
(working on RegisterTickerCommand)
Refactoring commands to provide a cleaner event-based API.
* removing dependencies between commands and domain model.
Refactoring commands to provide a cleaner event-based API.
* removing dependencies between commands and domain model.
Moved commands to exchange.infrastructure.adaptor.eventapi.
Added check to ensure market book is always empty.
Removed market order book from OrderBook.
Removed ticker field from OrderBook.
Replaced RegisterTickerCommand with RegisterCurrencyPairCommand.
Replaced TickerRegisteredEvent with CurrencyPairRegisteredEvent.
Replaced TickerRegisteredEvent with CurrencyPairRegisteredEvent.