[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