ShowMessageAsync recieving MaxHeight as optional parameter

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 the parameter MaxHeight to ShowMessageAsync this way, where the number **200** is the MaxHeight.:
```
await _metroWindow.ShowMessageAsync(title, message, style, settings, 200);
```

With this you can have an scrollbar even if the text's size is not bigger than the window's size
![image](https://cloud.githubusercontent.com/assets/1263856/4686303/f873ceb8-5643-11e4-8e24-3597a9a467fb.png)


Fixes #1451