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

.gitignore

@@ -51,6 +51,7 @@ unit-tests.log
 rat-results.txt
 scalastyle.txt
 conf/*.conf
+scalastyle-output.xml
 
 # For Hive
 metastore_db/

pom.xml

@@ -957,6 +957,30 @@
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-source-plugin</artifactId>
       </plugin>
+      <plugin>
+        <groupId>org.scalastyle</groupId>
+        <artifactId>scalastyle-maven-plugin</artifactId>
+        <version>0.4.0</version>
+        <configuration>
+          <verbose>false</verbose>
+          <failOnViolation>true</failOnViolation>
+          <includeTestSourceDirectory>false</includeTestSourceDirectory>
+          <failOnWarning>false</failOnWarning>
+          <sourceDirectory>${basedir}/src/main/scala</sourceDirectory>
+          <testSourceDirectory>${basedir}/src/test/scala</testSourceDirectory>
+          <configLocation>scalastyle-config.xml</configLocation>
+          <outputFile>scalastyle-output.xml</outputFile>
+          <outputEncoding>UTF-8</outputEncoding>
+        </configuration>
+        <executions>
+          <execution>
+            <phase>package</phase>
+            <goals>
+              <goal>check</goal>
+            </goals>
+          </execution>
+        </executions>
+      </plugin>
     </plugins>
   </build>