The API is still not very Java-friendly because `Array[Item]` in `freqItemsets` is recognized as `Object` in Java. We might want to define a case class to wrap the return pair to make it Java friendly.

Author: Xiangrui Meng <meng@databricks.com>

Closes #4661 from mengxr/SPARK-5519 and squashes the following commits:

58ccc25 [Xiangrui Meng] add user guide with example code for fp-growth