[SPARK-1683] Track task read metrics.

This commit adds a new metric in TaskMetrics to record
the input data size and displays this information in the UI.

An earlier version of this commit also added the read time,
which can be useful for diagnosing straggler problems,
but unfortunately that change introduced a significant performance
regression for jobs that don't do much computation. In order to
track read time, we'll need to do sampling.

The screenshots below show the UI with the new "Input" field,
which I added to the stage summary page, the executor summary page,
and the per-stage page.

![image](https://cloud.githubusercontent.com/assets/1108612/3167930/2627f92a-eb77-11e3-861c-98ea5bb7a1a2.png)

![image](https://cloud.githubusercontent.com/assets/1108612/3167936/475a889c-eb77-11e3-9706-f11c48751f17.png)

![image](https://cloud.githubusercontent.com/assets/1108612/3167948/80ebcf12-eb77-11e3-87ed-349fce6a770c.png)
I see. When I wrote the original test I just treated everything as a holder for ints and longs without regard for their actual meanings, but this is also fine.