Merge branch 'dev-checkpoint' of github.com:radlab/spark into dev-merge

Conflicts:
	core/src/main/scala/spark/ParallelCollection.scala
	core/src/main/scala/spark/RDD.scala
	core/src/main/scala/spark/rdd/BlockRDD.scala
	core/src/main/scala/spark/rdd/CartesianRDD.scala
	core/src/main/scala/spark/rdd/CoGroupedRDD.scala
	core/src/main/scala/spark/rdd/CoalescedRDD.scala
	core/src/main/scala/spark/rdd/FilteredRDD.scala
	core/src/main/scala/spark/rdd/FlatMappedRDD.scala
	core/src/main/scala/spark/rdd/GlommedRDD.scala
	core/src/main/scala/spark/rdd/HadoopRDD.scala
	core/src/main/scala/spark/rdd/MapPartitionsRDD.scala
	core/src/main/scala/spark/rdd/MapPartitionsWithSplitRDD.scala
	core/src/main/scala/spark/rdd/MappedRDD.scala
	core/src/main/scala/spark/rdd/PipedRDD.scala
	core/src/main/scala/spark/rdd/SampledRDD.scala
	core/src/main/scala/spark/rdd/ShuffledRDD.scala
	core/src/main/scala/spark/rdd/UnionRDD.scala
	core/src/main/scala/spark/scheduler/ResultTask.scala
	core/src/test/scala/spark/CheckpointSuite.scala