spark-suggest-box: use bootstrap styling on search box

Use bootstrap styling.
Fixes https://github.com/dart-lang/spark/issues/947

Review : @dinhviethoa
cc : @ussuri

![image](https://f.cloud.github.com/assets/756988/2058667/e69a6850-8b7d-11e3-8e63-7641f626a99c.png)