Add isCached field (inside an env) and unpersist().
Minor inline comment style fix.
Add `isCheckpointed` field and checkpoint().
First cut at PipelinedRDD.

- Add constructor & getJRDD logic
- Use PipelinedRDD in lapply and lapplyPartitionsWithIndex
Put serialized field inside env for both RDD and PipelinedRDD.
Add a multi-transformation test to benchmark on pipelining.
Minor edits per Shivaram's comments.
Add setCheckpointDir() to context.R; comment fix.

Conflicts:
	pkg/R/RDD.R
Add tests for actions on PipelinedRDD.
Remove outdated comment.
Review feedback.
Merge branch 'master' into pipelined

Conflicts:
	pkg/R/sparkR.R
Merge pull request #12 from concretevitamin/pipelined

First cut at RDD-pipelining implementation.