Remove the getStageInfo() method from SparkContext.

This method exposes the Stage objects, which are
private to Spark and should not be exposed to the
user.

This method was added in https://github.com/apache/spark/commit/01d77f329f5878b7c8672bbdc1859f3ca95d759d; ccing @squito here in case there's a good reason to keep this!

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #308 from kayousterhout/remove_public_method and squashes the following commits:

2e2f009 [Kay Ousterhout] Remove the getStageInfo() method from SparkContext.
[SPARK-1371] fix computePreferredLocations signature to not depend on underlying implementation

Change to Map and Set - not mutable HashMap and HashSet

Author: Mridul Muralidharan <mridulm80@apache.org>

Closes #302 from mridulm/master and squashes the following commits:

df747af [Mridul Muralidharan] Address review comments
17e2907 [Mridul Muralidharan] fix computePreferredLocations signature to not depend on underlying implementation