Switched orderbook to show depth and not totalValue

Here is what it looks like:

![screen shot 2014-10-06 at 5 52 19 pm](https://cloud.githubusercontent.com/assets/5728307/4535863/3901fd96-4dbc-11e4-9f4d-fc70cdf7051a.png)

Implements #911
This function is so small it might be more readable to inline ```toPriceLevel``` since it is never reused, and doesn't even create real priceLevels.