Introduce an API for showing a Metro MessageBox in a MetroWindow

A combination #550 and #657. I created a function on ```MetroWindow``` that allows you to asynchronously show a metro-style MessageBox **IN** that MetroWindow.

- [x] Basic functionality
- [ ] Localization friendly
- [ ] Animations!

![md2](https://f.cloud.github.com/assets/251501/1458333/84fbae32-4362-11e3-8772-80db188743bb.PNG)

Apart of #744