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

Yeah, that was a special case that I knew would have to change.

Before, it was `return $filter('rpamount')($rootScope.balance);` but Jed wanted it to show 0 decimal places and rpamount shows 6.

If we restructure AccountLines into a generic Balances service, then that would be optimal.