HOTFIX: Add no-arg SparkContext constructor in Java

Self explanatory.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #878 from pwendell/java-constructor and squashes the following commits:

2cc1605 [Patrick Wendell] HOTFIX: Add no-arg SparkContext constructor in Java

(cherry picked from commit b6d22af040073cd611b0fcfdf8a5259c0dfd854c)
Signed-off-by: Aaron Davidson <aaron@databricks.com>