Bug 938177 - Browser Settings Moved to System Settings

Duplicates browser app's settings in Settings. Browser app is apparently
being deleted, so modifications there likely do not matter.
This is going to be a bit of work, but I would like to move this out of bootstrap.js if possible. The logic in here is growing too much, and I think it's gotten to the point where this needs to be it's own file. Can you create a new file in: system/js/browser_settings.js which exports a singleton and is instantiated by bootstrap.js? Here is a rough wiring of how I think it should look:

```
// In bootstrap.js
window.browserSettings = new BrowserSettings();
window.browserSettings.start();
```

```
// In browser_settings.js
(function(exports) {

  function BrowserSettings() {}

  BrowserSettings.prototype = {
    addListener: function() {},
    start: function() {
      // Adds settings observers
    }
  };

  exports.BrowserSettings = BrowserSettings;

}(window));
```