Fix button insets and stacking algorithm

See issue #89 and issue #84.

Buttons:
![screenshot_2014-12-06-20-24-33](https://cloud.githubusercontent.com/assets/6379975/5330022/fa9bd514-7d85-11e4-8639-35461c1688cd.png)

Stacking:
![screenshot_2014-12-06-20-26-15](https://cloud.githubusercontent.com/assets/6379975/5330023/2ca771da-7d86-11e4-8003-fba5062f9f27.png)