SPARK-1833 - Have an empty SparkContext constructor.

This is nicer than relying on new SparkContext(new SparkConf())

Author: Patrick Wendell <pwendell@gmail.com>

Closes #774 from pwendell/spark-context and squashes the following commits:

ef9f12f [Patrick Wendell] SPARK-1833 - Have an empty SparkContext constructor.
(cherry picked from commit 65533c7ec03e7eedf5cd9756822863ab6f034ec9)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>