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