Set the color of GroupBox via TemplateBinding

Added a setter for Background and BorderBrush, so you can set the color of the groupbox without overriding the whole style.

```
<GroupBox  Header="Products" Background="Green" BorderBrush="Blue">
</GroupBox>
```

![GroupBoxColor](https://f.cloud.github.com/assets/3532342/252982/42c5129e-8bbe-11e2-9bca-65ef11937a79.PNG)

(Just an example, looks better with other colors ;-) )