Adds currency pair service, used for tracking favorites
Adds integration of CurrencyPair favorites to prototype
Documentation
Merge pull request #785 from stellar/fix-order-direction

Fix the OrderBook buy and sell directions
Merge pull request #787 from stellar/order-backend-2

Order backend 2 - CurrencyPairs Service
Trading form
The payable amount value must be a string
Separate amount values from currency pairs