Add scalastyle plugin.

scalastyle-config.xml is generated by `sbt scalastyle-generate-config`

`sbt scalastyle` command will check style.
Fix style errors.

Suppress the following warnings.
 - warning file=/home/seulgikim/workspace/beyond/app/controllers/Application.scala message=Public method must have explicit type line=8 column=6
 - warning file=/home/seulgikim/workspace/beyond/app/controllers/Application.scala message=File must end with newline character

Disable HeaderMatchesChecker to suppress the following warnings. This checker
will be enabled after discussion.
 - warning file=/home/seulgikim/workspace/beyond/app/controllers/Application.scala message=Header does not match expected text line=1