Add unpersist() to JavaDoubleRDD and JavaPairRDD.

This fixes a minor inconsistency where [unpersist() was only available on JavaRDD](https://mail-archives.apache.org/mod_mbox/incubator-spark-user/201310.mbox/%3CCE8D8748.68C0%25YannLuppo%40livenation.com%3E) and not JavaPairRDD / JavaDoubleRDD.   I also added support for the new optional `blocking` argument added in 0.8.

Please merge this into branch-0.8, too.