add first version of metro message box

Default MessageBox looks awkward. This one should help :)

Constructor is mostly the same as default System.Windows.MessageBox;

![image](https://cloud.githubusercontent.com/assets/1394349/5489081/e5e53138-86d6-11e4-892b-6158caa34f38.png)
![image](https://cloud.githubusercontent.com/assets/1394349/5489101/101ca274-86d7-11e4-87ec-8a73bc6cb50e.png)