This simplifies the shell a bunch and passes all arguments through to spark-submit.

There is a tiny incompatibility from 0.9.1 which is that you can't put `-c` _or_ `--cores`, only `--cores`. However, spark-submit will give a good error message in this case, I don't think many people used this, and it's a trivial change for users.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #542 from pwendell/spark-shell and squashes the following commits:

9eb3e6f [Patrick Wendell] Updating Spark docs
b552459 [Patrick Wendell] Andrew's feedback
97720fa [Patrick Wendell] Review feedback
aa2900b [Patrick Wendell] SPARK-1619 Launch spark-shell with spark-submit