[SPARK-5565][ML] LDA wrapper for Pipelines API

This adds LDA to spark.ml, the Pipelines API.  It follows the design doc in the JIRA: [https://issues.apache.org/jira/browse/SPARK-5565], with one major change:
* I eliminated doc IDs.  These are not necessary with DataFrames since the user can add an ID column as needed.

Note: This will conflict with [https://github.com/apache/spark/pull/9484], but I'll try to merge [https://github.com/apache/spark/pull/9484] first and then rebase this PR.

CC: hhbyyh feynmanliang  If you have a chance to make a pass, that'd be really helpful--thanks!  Now that I'm done traveling & this PR is almost ready, I'll see about reviewing other PRs critical for 1.6.

CC: mengxr

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #9513 from jkbradley/lda-pipelines.