[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.
Similarly, "[random]" can be removed here.
Also please change the indent of arguments in next lines to two tabs (see the style guide at: https://cwiki.apache.org/confluence/display/SPARK/Spark+Code+Style+Guide#SparkCodeStyleGuide-Indentation)