[SPARK-7475] [MLLIB] adjust ldaExample for online LDA

jira: https://issues.apache.org/jira/browse/SPARK-7475

Add a new argument to specify the algorithm applied to LDA, to exhibit the basic usage of LDAOptimizer.

cc jkbradley

Author: Yuhao Yang <hhbyyh@gmail.com>

Closes #6000 from hhbyyh/ldaExample and squashes the following commits:

0a7e2bc [Yuhao Yang] fix according to comments
5810b0f [Yuhao Yang] adjust ldaExample for online LDA