Fix settings screen on tablets

Hi again,
another little cosmetic fix. On tablets the settings screen (and therefore the first screen a user will see of the app) looks like this:
![screenshot_2014-01-03-19-26-49](https://f.cloud.github.com/assets/1621900/1904009/8fbf7870-7c89-11e3-9873-5dd31b145e4b.png)
Borders are visible left and right, while the content does not seem to fit into this box, especially the checkbox.

It seems to be a problem with using a single preferenceFragment inside a preferenceActivity. The official Settings guide states: ["You can add a PreferenceFragment to any activity—you don't need to use PreferenceActivity"][androidsettings]
Apparently it's even necessary not use a PreferenceActivity here.

I assume, an other solution would be not to use the PreferenceFragment, although the developer guide recommends to use it.

[androidsettings]: http://developer.android.com/guide/topics/ui/settings.html#Fragment