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

I would much prefer the entirety of this function look like:

```javascript
$scope.getCurrencyBalance = function(currency) {
   return Balances.get(currency);
}
```

`Balances.get` should return an Amount object (as described in structs.jsdoc.

Thoughts?