Make windows scalable by DPI

When you are using Windows under high DPI mode, the windows of WinSparkle was too small. Now it will be scaled by the DPI.

After:
![image](https://cloud.githubusercontent.com/assets/158528/4866198/03a1ac24-612a-11e4-9322-c229adcef74d.png)

Before:
![image](https://cloud.githubusercontent.com/assets/158528/4866177/e255c28a-6129-11e4-864d-15e87874adf2.png)