Add leftOuterJoin() and rightOuterJoin() to the RDD class.
Merge pull request #111 from hlin09/hlin09

Adds function foreach() and foreachPartition(). It might help benchmark functions.
[SPARKR-130] Add persist(storageLevel) API to RDD.
Update comment to add a reference to storage levels.
Fix per the review comments.
Merge pull request #115 from sun-rui/SPARKR-130

[SPARKR-130] Add persist(storageLevel) API to RDD.
Merge pull request #112 from sun-rui/outer_join

Add leftOuterJoin() and rightOuterJoin() to the RDD class.
Merge remote-tracking branch 'upstream/master' into hlin09
Minor fix.