Normalize offers from order book snapshot
Moves "open" order book management into OrderBook service

This commit moves the management of order book objects into order-book.js
from trading-service.js.  Most notably, this commit privatizes the
OrderBook constructor so we can centrally control instances.  In addition,
we have the basics stubbed out for updating order book in response to
network messages
Adds CurrencyPairs.invert()
Use CurrencyPair objects in Trading.getOrderBook
Adds Trading.offer.toFriendlyOffer

This commit adds the concept of a FriendlyOffer, that is, an offer that
is expressed as a buy/sell operation according to the current users
preference.  An example usage is provided on the trade page that
shows these friendly offers in the "My Offers" section
Fixes inverted order book bug
Merge pull request #796 from stellar/order-backend-3

Order backend 3 - FriendlyOffers