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.