Clean-up of Metrics Code/Docs and Add Ganglia Sink

Fix target JVM version in scala build

SPARK-660: Add StorageLevel support in Python

Provide docs to describe running on CDH/HDP cluster.

Adding Apache license to two files

0.8 Doc changes for make-distribution.sh

Fixed the bug that ResultTask was not properly deserializing outputId.

The sc.StorageLevel -> StorageLevel pathway is a bit janky, but otherwise
the shell would have to call a private method of SparkContext. Having
StorageLevel available in sc also doesn't seem like the end of the world.
There may be a better solution, though.

As for creating the StorageLevel object itself, this seems to be the best
way in Python 2 for creating singleton, enum-like objects:
http://stackoverflow.com/questions/36932/how-can-i-represent-an-enum-in-python

YARN build fixes

Docs describing Spark monitoring and instrumentation

This doc consolidates information relevant to CDH/HDP users in a single place.

SPARK-821: Don't cache results when action run locally on driver

It uses reflection... I am not proud of that fact, but it at least ensures
compatibility (sans refactoring of the StorageLevel stuff).