Adds two functions from Spark: distinct() and mapValues().

Starting with SparkR by adding two functions from Spark:
distinct(): creates a new RDD with distinct elements from a given RDD.
mapValues(): applies a transformation to the values of pairwise RDD without changing the keys.
Thanks Shivaram for the comments. I am trying to use the current number of partitions as default value in distinct. Please let me know whether that involves too much complexity.