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)

This section:

```javascript
if(!sections.currencies || !sections.currencies.length) {
  $scope.currencies = [];
  $scope.searchStatus = 'no_currencies';
  return;
}

$scope.searchStatus = 'found';

$scope.currencies = sections.currencies.map(function(currency) {
  var parts = currency.split(/\s+/, 2);

  return {
    currency: parts[0],
    issuer: parts[1]
  }
});
```

could be changed to this:

```javascript
$scope.currencies = _.map(sections.currencies, function(currency) {
  var parts = currency.split(/\s+/, 2);

  return {
    currency: parts[0],
    issuer: parts[1]
  }
});

$scope.searchStatus = _.any($scope.currencies) ? 'found' : 'no_currencies';

```

lodash's map will work with undefined and null values.  Not strictly necessary, just giving you an FYI