Added a MetroPopup that can also be used like a Message Box

Added a MetroPopup that can also be used like a Message Box.

You can use it as an XAML element like this:

```
<Controls:MetroPopup x:Name="TextBoxPopup"
    Header="Input Text"
    ButtonOrientation="Horizontal"
    PopupButton="OKCancel"
    Closed="TextBoxPopup_OnClosed">

    <TextBox Width="400"/>
</Controls:MetroPopup>
```

And open it from code behinde with:

```
TextBoxPopup.IsOpen = true;

// or

TextBoxPopup.ShowDialog();
```

![2013-06-17 15-12-14_mahapps metro demo](https://f.cloud.github.com/assets/3532342/662311/95305c20-d752-11e2-925d-3edb8e9062b3.png)

Or you can use it MessageBox like with this from anywhere in the code (this will temporary make the Window uncloseable and will be in front of everything else through the AdornerLayer):

```
var result = MetroPopup.Show("This is a Test Message Popup", "Popup", PopupButton.YesNoCancel, PopupImage.Information);
```

![2013-06-17 15-11-39_mahapps metro demo](https://f.cloud.github.com/assets/3532342/662312/952ea330-d752-11e2-83a2-3a8be34a8669.png)