metro circle button demo fix

fixes the button demo

workaround

```xml
<Rectangle Width="20"
           Height="20"
           Fill="{DynamicResource BlackBrush}">
    <Rectangle.OpacityMask>
        <VisualBrush Stretch="Fill"
                     Visual="{DynamicResource appbar_city}" />
    </Rectangle.OpacityMask>
</Rectangle>
```

it's not MahApps.Metro issue...

http://social.msdn.microsoft.com/Forums/vstudio/en-US/7e9e0817-9cc3-497a-b21f-dc69e7b946cb/visualbrush-not-updated-when-we-change-palette?forum=wpf

http://connect.microsoft.com/VisualStudio/feedback/details/633535/msdn-forum-dynamicresource-in-nested-resource-dictionary-not-working

![6nx28giopkoql0ywrknurx6yq5d1y](https://f.cloud.github.com/assets/658431/2369110/2b80909e-a7cf-11e3-86b2-3c5f6341f2fd.jpg)