Cleanup spark-button widget

- Fixes TODO from `SparkButton.dart`
- Rewrite example for `<spark-button>`

About the example, I removed `example_ui.html/dart`
Because I don't think `<polymer-element name="example-ui" />` is necessary. if you don't agree, please advicse me.

review @ussuri

![image](https://f.cloud.github.com/assets/756988/2162960/a6e2ef7a-94d8-11e3-9523-b331817ad5fe.png)