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)

I set this to 6 digits for now. I talked to Jed and currencies should use the ```max_decimal_places``` setting, but prices and amounts should use signifigant digits to avoid rounding to 0.