Job killing

Moving https://github.com/mesos/spark/pull/935 here

The high level idea is to have an "interrupted" field in TaskContext, and a task should check that flag to determine if its execution should continue. For convenience, I provide an InterruptibleIterator which wraps around a normal iterator but checks for the interrupted flag. I also provide an InterruptibleRDD that wraps around an existing RDD.

As part of this pull request, I added an AsyncRDDActions class that provides a number of RDD actions that return a FutureJob (extending scala.concurrent.Future). The FutureJob can be used to kill the job execution, or waits until the job finishes.

This is NOT ready for merging yet. Remaining TODOs:

1. Add unit tests
2. Add job killing functionality for local scheduler (current job killing functionality only works in cluster scheduler)