[SPARK-4017] show progress bar in console

The progress bar will look like this:

![1___spark_job__85_250_finished__4_are_running___java_](https://cloud.githubusercontent.com/assets/40902/4854813/a02f44ac-6099-11e4-9060-7c73a73151d6.png)

In the right corner, the numbers are: finished tasks, running tasks, total tasks.

After the stage has finished, it will disappear.

The progress bar is only showed if logging level is WARN or higher (but progress in title is still showed), it can be turned off by spark.driver.showConsoleProgress.


This output is a little confusing:

```
    ] 1834 + 4 / 1000
```

What about adding parentheses to make the associativity more clear?

```
    ] (1834 + 4) / 1000
```