[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.
Just found that hadoop-common depends on commons-math3-3.1.1 since v2.3.0 (HADOOP-9594). 3.3 is not compatible with 3.1.1. So there is (or will be) a problem no matter which version we include. Do we have alternative solutions?