[SPARK-4588] ML Attributes

This continues the work in #4460 from srowen . The design doc is published on the JIRA page with some minor changes.

Short description of ML attributes: https://github.com/apache/spark/pull/4925/files?diff=unified#diff-95e7f5060429f189460b44a3f8731a35R24

More details can be found in the design doc.

srowen Could you help review this PR? There are many lines but most of them are boilerplate code.

Author: Xiangrui Meng <meng@databricks.com>
Author: Sean Owen <sowen@cloudera.com>

Closes #4925 from mengxr/SPARK-4588-new and squashes the following commits:

71d1bd0 [Xiangrui Meng] add JavaDoc for package ml.attribute
617be40 [Xiangrui Meng] remove final; rename cardinality to numValues
393ffdc [Xiangrui Meng] forgot to include Java attribute group tests
b1aceef [Xiangrui Meng] more tests
e7ab467 [Xiangrui Meng] update ML attribute impl
7c944da [Sean Owen] Add FeatureType hierarchy and categorical cardinality
2a21d6d [Sean Owen] Initial draft of FeatureAttributes class