Add maximum() and minimum() API to RDD.

They are named maximum() and minimum() to avoid conflict with R's max() and min() functions.
Oops sorry I meant RRDD's own `reduce()` [1]. Using this would seem to eliminate some duplicated code.

[1] https://github.com/amplab-extras/SparkR-pkg/blob/master/pkg/R/RDD.R#L544