Add JavaAPICompletenessChecker.

This is used to find methods in the Scala API that
need to be ported to the Java API.  To use it:

  ./run spark.tools.JavaAPICompletenessChecker
Remove annotation code that broke build.
Merge pull request #713 from JoshRosen/java-api-completeness-checker

Add JavaAPICompletenessChecker script to find methods that are missing from the Java API