[SPARK-2130] End-user friendly String repr for StorageLevel in Python

JIRA issue https://issues.apache.org/jira/browse/SPARK-2130

This PR adds an end-user friendly String representation for StorageLevel
in Python, similar to ```StorageLevel.description``` in Scala.
```
>>> rdd = sc.parallelize([1,2])
>>> storage_level = rdd.getStorageLevel()
>>> storage_level
StorageLevel(False, False, False, False, 1)
>>> print(storage_level)
Serialized 1x Replicated
```

Author: Kan Zhang <kzhang@apache.org>

Closes #1096 from kanzhang/SPARK-2130 and squashes the following commits:

7c8b98b [Kan Zhang] [SPARK-2130] Prettier epydoc output
cc5bf45 [Kan Zhang] [SPARK-2130] End-user friendly String representation for StorageLevel in Python
SPARK-1990: added compatibility for python 2.6 for ssh_read command

https://issues.apache.org/jira/browse/SPARK-1990

There were some posts on the lists that spark-ec2 does not work with Python 2.6. In addition, we should check the Python version at the top of the script and exit if it's too old

Author: Anant <anant.asty@gmail.com>

Closes #941 from anantasty/SPARK-1990 and squashes the following commits:

4ca441d [Anant] Implmented check_optput withinthe module to work with python 2.6
c6ed85c [Anant] added compatibility for python 2.6 for ssh_read command
SPARK-2035: Store call stack for stages, display it on the UI.

I'm not sure about the test -- I get a lot of unrelated failures for some reason. I'll try to sort it out. But hopefully the automation will test this for me if I send a pull request :).

I'll attach a demo HTML in [Jira](https://issues.apache.org/jira/browse/SPARK-2035).

Author: Daniel Darabos <darabos.daniel@gmail.com>
Author: Patrick Wendell <pwendell@gmail.com>

Closes #981 from darabos/darabos-call-stack and squashes the following commits:

f7c6bfa [Daniel Darabos] Fix bad merge. I undid 83c226d454 by Doris.
3d0a48d [Daniel Darabos] Merge remote-tracking branch 'upstream/master' into darabos-call-stack
b857849 [Daniel Darabos] Style: Break long line.
ecb5690 [Daniel Darabos] Include the last Spark method in the full stack trace. Otherwise it is not visible if the stage name is overridden.
d00a85b [Patrick Wendell] Make call sites for stages non-optional and well defined
b9eba24 [Daniel Darabos] Make StageInfo.details non-optional. Add JSON serialization code for the new field. Verify JSON backward compatibility.
4312828 [Daniel Darabos] Remove Mima excludes for CallSite. They should be unnecessary now, with SPARK-2070 fixed.
0920750 [Daniel Darabos] Merge remote-tracking branch 'upstream/master' into darabos-call-stack
a4b1faf [Daniel Darabos] Add Mima exclusions for the CallSite changes it has picked up. They are private methods/classes, so we ought to be safe.
932f810 [Daniel Darabos] Use empty CallSite instead of null in DAGSchedulerSuite. Outside of testing, this parameter always originates in SparkContext.scala, and will never be null.
ccd89d1 [Daniel Darabos] Fix long lines.
ac173e4 [Daniel Darabos] Hide "show details" if there are no details to show.
6182da6 [Daniel Darabos] Set a configurable limit on maximum call stack depth. It can be useful in memory-constrained situations with large numbers of stages.
8fe2e34 [Daniel Darabos] Store call stack for stages, display it on the UI.