Orderbook Summary backend

this PR adds `OrderBook#getSummary` method, as well as an unfinished module of the summary display on the trading page.
I knew there was a reason the implicit constructor and this usage in ```FriendlyOffers``` smelt funny. The problem is that the capitalization of the name and the interface suggest that it is a class with static methods. Under the hood it is really an object instance when it shouldn't be.