Use Bootstrap progress bars in web UI

This uses the [Twitter Bootstrap progress bars] (http://twitter.github.io/bootstrap/components/#progress) in the Web UI instead of SVG, solving [SPARK-820] (https://spark-project.atlassian.net/browse/SPARK-820).
![stage progress bars](https://f.cloud.github.com/assets/4754931/874606/f1a04f12-f88e-11e2-82ff-dcb277522585.png)