Basic concept of glow window from GitHub project

Added a new property for MetroWindow called the GlowBrushProperty. This can be set to a solid color brush which will be used for the drop shadow effect on the metro window.

Original idea was from https://github.com/Grabacr07/MetroLikeWindow

![Glow](https://f.cloud.github.com/assets/2020447/162985/863638be-77e6-11e2-9369-9ac733dcd452.png)