[SPARK-7582] [MLLIB] user guide for StringIndexer

This PR adds a Java unit test and user guide for `StringIndexer`. I put it before `OneHotEncoder` because they are closely related. jkbradley

Author: Xiangrui Meng <meng@databricks.com>

Closes #6561 from mengxr/SPARK-7582 and squashes the following commits:

4bba4f1 [Xiangrui Meng] fix example
ba1cd1b [Xiangrui Meng] fix style
7fa18d1 [Xiangrui Meng] add user guide for StringIndexer
136cb93 [Xiangrui Meng] add a Java unit test for StringIndexer