This PR updates `genjavadocSettings` to use a patched version of `genjavadoc-plugin` that hides package private classes/methods/interfaces in the generated Java API doc. The patch can be found at: https://github.com/typesafehub/genjavadoc/compare/master...mengxr:spark-1.4.

It wasn't merged into the main repo because there exist corner cases where a package private Scala class has to be a Java public class in order to compile. This doesn't seem to apply to the Spark codebase. So we release a patched version under `org.spark-project` and use it in the Spark build. brkyvz is publishing the artifacts to Maven Central.

Need more people audit the generated APIs and make sure we don't have false negatives.

Current listed classes under `org.apache.spark.rdd`:
![screen shot 2015-05-29 at 12 48 52 pm](https://cloud.githubusercontent.com/assets/829644/7891396/28fb9daa-0601-11e5-8ed8-4e9522d25a71.png)

After this PR:
![screen shot 2015-05-29 at 12 48 23 pm](https://cloud.githubusercontent.com/assets/829644/7891408/408e210e-0601-11e5-975c-ff0a02eb5c91.png)

cc: pwendell rxin srowen

Author: Xiangrui Meng <meng@databricks.com>

Closes #6506 from mengxr/SPARK-5610 and squashes the following commits:

489c785 [Xiangrui Meng] update genjavadocSettings to use the patched version of genjavadoc