Made MetroCircleToggleButtonStyle's foreground GrayBrush6 when it is disabled

ToggleButtons implementing MetroCircleToggleButtonStyle will now have their contents grayed out when disabled. (If the contents bind to the toggle button's foreground)

![untitled](https://f.cloud.github.com/assets/574734/2462678/c33045d6-af83-11e3-8584-33415e0f6a54.png)

Changes made in addition to 41b8d657b7035e5ef44337d875b3fb275a1032f7.