Adds $get global helper
Adds FriendlyOffers service
Implements the first revision of the OrderBook price levels system

This commit adds a non-updating order book to the bottom of the trade page,
and the corresponding backend for use in the final trading page.

Notably, we are now properly producing "pricingLevel" data for an order
books snapshot, as well as emitting trading:order-book-updated events
when new data arrives.
Update error message
Update order book data for invalid currency pairs
Merge pull request #801 from stellar/order-backend-4

Order backend 4 - Towards a live updating order book