SPARK-2651: Add maven scalastyle plugin

Can be run as: "mvn scalastyle:check"

Author: Rahul Singhal <rahul.singhal@guavus.com>

Closes #1550 from rahulsinghaliitd/SPARK-2651 and squashes the following commits:

53748dd [Rahul Singhal] SPARK-2651: Add maven scalastyle plugin