SPARK-1467: Make StorageLevel.apply() factory methods Developer APIs

We may want to evolve these in the future to add things like SSDs, so let's mark them as experimental for now. Long-term the right solution might be some kind of builder. The stable API should be the existing StorageLevel constants.

Author: Sandeep <sandeep@techaddict.me>

Closes #551 from techaddict/SPARK-1467 and squashes the following commits:

6bdda24 [Sandeep] SPARK-1467: Make StorageLevel.apply() factory methods as Developer Api's We may want to evolve these in the future to add things like SSDs, so let's mark them as experimental for now. Long-term the right solution might be some kind of builder. The stable API should be the existing StorageLevel constants.
SPARK-1606: Infer user application arguments instead of requiring --arg.

This modifies spark-submit to do something more like the Hadoop `jar`
command. Now we have the following syntax:

./bin/spark-submit [options] user.jar [user options]

Author: Patrick Wendell <pwendell@gmail.com>

Closes #563 from pwendell/spark-submit and squashes the following commits:

32241fc [Patrick Wendell] Review feedback
3adfb69 [Patrick Wendell] Small fix
bc48139 [Patrick Wendell] SPARK-1606: Infer user application arguments instead of requiring --arg.
merge mastet