Added a pair-not-selected state on the trading balances widget

PR for issue https://github.com/stellar/stellar-client/issues/883

What it looks like in action:
![screen shot 2014-09-29 at 12 03 04 pm](https://cloud.githubusercontent.com/assets/5728307/4447533/7a3b960c-480b-11e4-98b6-63cbc9952bed.png)