Updated Spark version to 1.0.0

Also adding the fastutil dependency back in. Spark 0.9 had included the dependency on
fastutil, which we also depended on! But Spark 1.0 apparently removes that dependency.
So this commit adds that back in, for our own use.
Updating all instances of Seq[_] to Iterable[_] in RDD.groupBy()

Apparently, the signature for the RDD.groupBy method has changed (in 1.0) to return an Iterable
rather than a Seq.  This commit includes all the changes taht are needed to account for this change
downstream in our code, mostly updating types to Iterable and inserting a few calls to toSeq in cases
where that's not sufficient.