Message Dialog receiving MinHeight and MaxHeight in MetroDialogSettings

Currently when a message is too long, bigger than the window, the content of the messsage is lost.
This message has 61 lines and you can only see the first 20 or so.
![image](https://cloud.githubusercontent.com/assets/1263856/4686241/2d682c00-5643-11e4-9189-a5d29961d2c2.png)

With this pull request, if the message is too long, an scrollbar will appear.
![image](https://cloud.githubusercontent.com/assets/1263856/4686267/860aba9e-5643-11e4-8157-921c4370ed7d.png)

If the message is short, it will not appear
![image](https://cloud.githubusercontent.com/assets/1263856/4686276/9a1a2f38-5643-11e4-81cc-b6d647ec2724.png)

Optionally you can pass MinHeightPercent and MaxHeightPercent as parameters of **MetroDialogSettings**:
```
var settings = new MetroDialogSettings
            {
                ColorScheme = MetroDialogColorScheme.Theme,
                AnimateShow = false,
                AnimateHide = false,
                MinHeightPercent = 0.25,
                MaxHeightPercent = 0.75
            };

var result = await _metroWindow.ShowMessageAsync("Title", "Message", style, settings);
```

With this you can have an scrollbar even if the text's size is not bigger than the window's size. 
Min and Max values are received as percents of the Windows Size instead of Pixels
![image](https://cloud.githubusercontent.com/assets/1263856/4709094/ef320184-589c-11e4-9c49-c4168a8a310f.png)

Fixes #1451