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
BUILD: Add more content to make-distribution.sh.
refactor Suite using taskScheduler.resourceOffer
add basic resourceOffer Test
refactor Suite using taskScheduler.resourceOffer
add basic resourceOffer Test
add more tests for TaskSchedulerImpl