Expose numPartitions parameter in JavaPairRDD.sortByKey()

This change make Java and Scala API on sortByKey() the same.
Use braces to shorten the line.
Leave default value of numPartitions to Scala code.
Hook directly to Scala API
Merge pull request #249 from ngbinh/partitionInJavaSortByKey

Expose numPartitions parameter in JavaPairRDD.sortByKey()

This change makes Java and Scala API on sortByKey() the same.