Make RDD API private in SparkR for Spark 1.4

This ticket proposes making the RDD API in SparkR private for the 1.4 release. The motivation for doing so are discussed in a larger design document aimed at a more top-down design of the SparkR APIs. A first cut that discusses motivation and proposed changes can be found at http://goo.gl/GLHKZI

The main points in that document that relate to this ticket are:

• The RDD API requires knowledge of the distributed system and is pretty low level. This is not very suitable for a number of R users who are used to more high-level packages that work out of the box.
• The RDD implementation in SparkR is not fully robust right now: we are missing features like spilling for aggregation, handling partitions which don't fit in memory etc. There are further limitations like lack of hashCode for non-native types etc. which might affect user experience.

The only change we will make for now is to not export the RDD functions as public methods in the SparkR package and I will create another ticket for discussing more details public API for 1.5