SPARK-1973. Add randomSplit to JavaRDD (with tests, and tidy Java tests)

I'd like to use randomSplit through the Java API, and would like to add a convenience wrapper for this method to JavaRDD. This is fairly trivial. (In fact, is the intent that JavaRDD not wrap every RDD method? and that sometimes users should just use JavaRDD.wrapRDD()?)

Along the way, I added tests for it, and also touched up the Java API test style and behavior. This is maybe the more useful part of this small change.

Author: Sean Owen <sowen@cloudera.com>
Author: Xiangrui Meng <meng@databricks.com>

This patch had conflicts when merged, resolved by
Committer: Xiangrui Meng <meng@databricks.com>

Closes #919 from srowen/SPARK-1973 and squashes the following commits:

148cb7b [Sean Owen] Some final Java test polish, while we are at it
1fc3f3e [Xiangrui Meng] more cleaning on Java 8 tests
9ebc57f [Sean Owen] Use accumulator instead of temp files to test foreach
5efb0be [Sean Owen] Add Java randomSplit, and unit tests (including for sample)
5dcc158 [Sean Owen] Simplified Java 8 test with new language features, and fixed the name of MLB's greatest team
91a1769 [Sean Owen] Touch up minor style issues in existing Java API suite test