Made MetroCircleToggleButtonStyle use accent color when toggled. Fixes #963

When checked, ToggleButton using the MetroCircleToggleButtonStyle will turn to the AccentColorBrush instead of turning gray as show in the image below. Fixes #963.

In order to make sure that the content turns white when the toggle button is checked, just bind the glyph's foreground / fill / whatever is used as content to the ToggleButton's foreground.

For example:

```xml
<!-- Take note of the Rectangle's Fill. It is bound to the control's foreground -->
<Rectangle Width="20"
        Height="20"
        Fill="{Binding Path=Foreground, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ToggleButton}}}">
    <Rectangle.OpacityMask>
        <VisualBrush Stretch="Fill"
                    Visual="{DynamicResource appbar_city}"/>
    </Rectangle.OpacityMask>
</Rectangle>
```
## Before:

![image](https://f.cloud.github.com/assets/574734/2462221/28a3359a-af7f-11e3-9c7d-f9e6897505e1.png)

## After:
![image](https://f.cloud.github.com/assets/574734/2462128/4fb40f0c-af7e-11e3-8eb7-411c9d55a9ec.png)