Manage currencies

Adds a manage currencies button to the "all balances" pane.
Adds the "manage currencies" pane that allows searching for gateways, trusting their currencies, and removing trust from previous gateways.

Fixes #662.

![screen shot 2014-08-20 at 8 07 14 pm](https://cloud.githubusercontent.com/assets/3108007/3991386/e863426e-28e0-11e4-83f1-1bab4a869022.png)

![screen shot 2014-08-20 at 8 11 06 pm](https://cloud.githubusercontent.com/assets/3108007/3991389/f03384ae-28e0-11e4-8ec0-b7860fdbdbd4.png)

There will be a pass that updates the copy and fills in links that don't exist yet.