[SPARK-7547] [ML] Scala Example code for ElasticNet

This is scala example code for both linear and logistic regression. Python and Java versions are to be added.

Author: DB Tsai <dbt@netflix.com>

Closes #6576 from dbtsai/elasticNetExample and squashes the following commits:

e7ca406 [DB Tsai] fix test
6bb6d77 [DB Tsai] fix suite and remove duplicated setMaxIter
136e0dd [DB Tsai] address feedback
1ec29d4 [DB Tsai] fix style
9462f5f [DB Tsai] add example