First cut at RDD-pipelining implementation.

- Add a `PipelinedRDD` class (subclass of `RDD`); still using the S4 system
- Use `getJRDD()` to fetch an RDD's JavaRDD reference (previously was using `@jrdd`)
- Have `lapply()` and `lapplyPartitionsWithIndex()` use PipelinedRDD 

Note that this PR cherry-picked three commits out of the four in [#1](https://github.com/amplab-extras/SparkR-pkg/pull/1), so it'd be good to merge that first.