Order backend 4 - Towards a live updating order book

In this thrilling chapter of "Scott works on the trading client" we see that our hero has finally arrived at a point where an order book is _displayed on the screen_.

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This PR:

- adds the global function `$get` to make our lives easier to retrieve angular components in a repl
- Fleshes out the notion of  a `PriceLevel` struct, using it to drive the order book display
- Moves the creation of FriendlyOffer structs into the FriendlyOffers service
- Adds the rootScope broadcast `trading:order-book-updated`, to be used by controllers to get notification that an order book has changed

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Note, there is a private function called `mergePriceLevels` introduced in this PR that really shows the flaws in our "plain old javascript objects" strategy.  It has to do quite a number of flips to sum/merge the priceLevel data and spit it back out as a valid priceLevel object.

Its not perfect, but it will let us make forward progress for now.  As we rethink our strategy it will get cleaned up.
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