[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)

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #962 from kayousterhout/read_metrics and squashes the following commits:

f13b67d [Kay Ousterhout] Correctly format input bytes on executor page
8b70cde [Kay Ousterhout] Added comment about potential inaccuracy of bytesRead
d1016e8 [Kay Ousterhout] Udated SparkListenerSuite test
8461492 [Kay Ousterhout] Miniscule style fix
ae04d99 [Kay Ousterhout] Remove input metrics for parallel collections
719f19d [Kay Ousterhout] Style fixes
bb6ec62 [Kay Ousterhout] Small fixes
869ac7b [Kay Ousterhout] Updated Json tests
44a0301 [Kay Ousterhout] Fixed accidentally added line
4bd0568 [Kay Ousterhout] Added input source, renamed Hdfs to Hadoop.
f27e535 [Kay Ousterhout] Updates based on review comments and to fix rebase
bf41029 [Kay Ousterhout] Updated Json tests to pass
0fc33e0 [Kay Ousterhout] Added explicit backward compatibility test
4e52925 [Kay Ousterhout] Added Json output and associated tests.
365400b [Kay Ousterhout] [SPARK-1683] Track task read metrics.