Set icon background color for Windows 8 tiles

Windows 8.1 automatically determines the background color, which is not always sensible.
The background is also orange, which gives no contrast to the main content.
![win8gittile1](https://cloud.githubusercontent.com/assets/791115/5517210/f4c20e56-8910-11e4-93fc-adecdf239d11.png)

With this patch, a fixed background color is set. This makes the main content more clear.
![win8gittile2](https://cloud.githubusercontent.com/assets/791115/5517211/f882bad6-8910-11e4-9132-fd543ed570be.png)