[SPARK-2082] stratified sampling in PairRDDFunctions that guarantees exact sample size

Implemented stratified sampling that guarantees exact sample size using ScaRSR with two passes over the RDD for sampling without replacement and three passes for sampling with replacement.
The name is a little confusing, especially there is an argument named `counts`. See the comments below on the name of `Stratum`. If we change `Stratum` to `AcceptanceResult`, this could be changed to `getAcceptanceResults`.