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Remove outdated comment.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 pull request #23 from shivaram/master

Fixes for using SparkR with Hadoop2.
Merge branch 'master' into pipelined

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

First cut at RDD-pipelining implementation.
Merge pull request #24 from shivaram/master

Add a maven build file for SparkR
Fix isRdd() to properly check for class

The old approach, class(obj) == "RDD" returns a vector as long
as class attribute of obj.  For example:
> obj <- structure(list(), class = c("classA", "classB"))
> class(obj) == "RDD"
[1] FALSE FALSE
This causes getDependencies to fail.  Changed to:
inherits(obj, "RDD")
Merge pull request #26 from hafen/master

Fix isRdd() to properly check for class
Use inherits() to test an object's class.
Merge pull request #28 from concretevitamin/master

Use inherits() to test an object's class.
Update README with maven proxy instructions