SPARK-1801. expose InterruptibleIterator and TaskKilledException in deve...

...loper api

Author: Koert Kuipers <koert@tresata.com>

Closes #764 from koertkuipers/feat-rdd-developerapi and squashes the following commits:

8516dd2 [Koert Kuipers] SPARK-1801. expose InterruptibleIterator and TaskKilledException in developer api

(cherry picked from commit b22952fa1f21c0b93208846b5e1941a9d2578c6f)
Signed-off-by: Aaron Davidson <aaron@databricks.com>