Trading balance widget

Added a new widget that shows the user's current balance for the pairs they have selected.

![screen shot 2014-09-23 at 4 08 53 pm](https://cloud.githubusercontent.com/assets/5728307/4381460/a205e7da-4376-11e4-946e-c37de510cd1f.png)

https://github.com/stellar/stellar-client/issues/849

We could do that but that would mean that the Balances service would have to maintain an initial state which Jared said that we should not do in lieu of having the balances saved locally in each controller.

Balances.get should return an Amount object but the dashboard currently does not expect an Amount object

@deckar01 can you chime in?