[SPARK-7584] [MLLIB] User guide for VectorAssembler

This PR adds a section in the user guide for `VectorAssembler` with code examples in Python/Java/Scala. It also adds a unit test in Java.

jkbradley

Author: Xiangrui Meng <meng@databricks.com>

Closes #6556 from mengxr/SPARK-7584 and squashes the following commits:

11313f6 [Xiangrui Meng] simplify Java example
0cd47f3 [Xiangrui Meng] update user guide
fd36292 [Xiangrui Meng] update Java unit test
ce61ca0 [Xiangrui Meng] add Java unit test for VectorAssembler
e399942 [Xiangrui Meng] scala/python example code