Show correct precision for dashboard balances

This PR is in the trading branch because it needs the roundAmount filter on the balances. Can be ported to master but it was simpler to put it here since trading will be merged into master soon anyways.

This PR now makes the precision show according to `currencies.js`.
![screen shot 2014-10-03 at 4 48 25 pm](https://cloud.githubusercontent.com/assets/5728307/4513947/26de61d8-4b58-11e4-95bd-35123542ee70.png)

https://github.com/stellar/stellar-client/issues/912