Added a MetroTreeView and fixed #640

Added a MetroTreeView based on ListBox styles and fixed Issue https://github.com/MahApps/MahApps.Metro/issues/640
![2013-09-03 13_40_29-mahapps metro demo](https://f.cloud.github.com/assets/3532342/1072309/b65fcb42-148d-11e3-86f5-09abe5976df3.png)