Order book tab

Add the order book tab that shows a table of bids and asks for the selected currency pair.
When the currency pair is invalid nothing is shown.
When the currency pair is valid, but there are no orders, the tables are empty.

Part of #765.

![screen shot 2014-09-15 at 1 35 49 pm](https://cloud.githubusercontent.com/assets/3108007/4277587/464fc9c2-3d10-11e4-90a2-04858e9e4621.png)

so, we could then get rid of the explicit `$scope.loadOrderBookData();` in the controller, and let the watch statement unconditionally `loadOrderBookData`, correct?