[SPARK-4145] [WIP] Web UI job pages

This PR adds two new pages to the Spark Web UI:

- A jobs overview page, which shows details on running / completed / failed jobs.
- A job details page, which displays information on an individual job's stages.

The jobs overview page is now the default UI homepage; the old homepage is still accessible at `/stages`.

### Screenshots

#### New UI homepage

![image](https://cloud.githubusercontent.com/assets/50748/5119035/fd0a69e6-701f-11e4-89cb-db7e9705714f.png)

#### Job details page

(This is effectively a per-job version of the stages page that can be extended later with other things, such as DAG visualizations)

![image](https://cloud.githubusercontent.com/assets/50748/5119053/52ae737e-7020-11e4-9fe3-6aab694609ec.png)

### Key changes in this PR

- Rename `JobProgressPage` to `AllStagesPage`
- Expose `StageInfo` objects in the ``SparkListenerJobStart` event; add backwards-compatibility tests to JsonProtocol.
- Add additional data structures to `JobProgressListener` to map from stages to jobs.
- Add several fields to `JobUIData`.

I also added ~150 lines of Selenium tests as I uncovered UI issues while developing this patch.

### Limitations

If a job contains stages that aren't run, then its overall job progress bar may be an underestimate of the total job progress; in other words, a completed job may appear to have a progress bar that's not at 100%.

If stages or tasks fail, then the progress bar will not go backwards to reflect the true amount of remaining work.
I realize we use `"Unknown"` in a few places. Can you declare a
```
val UNKNOWN: String = "Unknown"
```
in `UIUtils`?