Add new function of union().
Add reserialize() before union if two RDDs are not both serialized.
Merge remote-tracking branch 'upstream/master' into hlin09
Make reserialize() private function in package.
minor style fix.
Remove redundant code in test.
Merge pull request #97 from hlin09/hlin09

Add function union(), with reserialize() to convert RDD to R serialized form.