SPARK-1815. SparkContext should not be marked DeveloperApi

Author: Sandy Ryza <sandy@cloudera.com>

Closes #753 from sryza/sandy-spark-1815 and squashes the following commits:

957a8ac [Sandy Ryza] SPARK-1815. SparkContext should not be marked DeveloperApi
(cherry picked from commit 2792bd016af2a67848e6f403c4e1e05e9f3e3c2a)

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