Initial Spark Support (Port of Scalding)

[tixxit]

This isn't merge ready yet, but since I know a few other people are interested (eg @non), I figured I'd make the PR now so I can get some more eyes on it.

A few thing worth mentioning:

• Spark doesn't really have support for something like Execution, so we just use its strict Unit-happy API everywhere.
• We only write to files when explicitly asked and use .cache() for memoizing the trees in-between steps.
• Basically just implements expandTimes and expandInMemory, but not prune or validate (not hard, just haven't done it).

[tixxit]

You can run the example with something like: SPARK_HOME=/path/to/spark ./iris-spark

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