Renamed TradeableItemPair to CurrencyPair
Updated CurrencyPair to compose two currencies rather than a TradeableItem and Currency.
Continued refactoring toward CurrencyPair.
Added docs to CurrencyPair
Refactored Orders.
Refactoring.

Added placeOrder to Exchange.
Added CurrencyPair field to OrderBook.
Removed Security (replaced by CurrencyPair).
Renamed CreateSecurityCommand to RegisterCurrencyPairCommand.
Renamed SecurityCreatedEvent to CurrencyPairRegisteredEvent.
Cleaned up RegisterCurrencyPairCommand and CurrencyPairRegisteredEvent.
Introduced currencyPair into BuyOrder and SellOrder.
Started on QuoteTest.
Merge branch 'refactor_core_domain' into develop