Created a circular button style for the ToggleButton.

Fixes #524

![t1](https://f.cloud.github.com/assets/251501/629776/e95e885a-d16f-11e2-897b-9f528de3ff36.png)
![t2](https://f.cloud.github.com/assets/251501/629777/e95ea524-d16f-11e2-9da2-a180a6f88e03.png)