SPARK-2651: Add maven scalastyle plugin

Can be run as: "mvn scalastyle:check"
Yes, `mvn package` is supposed to run these checks. I am surprised to hear that it didn't work for you. I had updated the PR yesterday, do by chance happen to have the older version (it was missing the line `<phase>package</phase>`)? Here is the snippet from my machine:

[INFO] --- maven-jar-plugin:2.4:jar (default-jar) @ spark-core_2.10 ---
[INFO] Building jar: /mnt/devel/rahuls/code/apache-spark/core/target/spark-core_2.10-1.1.0-SNAPSHOT.jar
[INFO] 
[INFO] --- maven-site-plugin:3.3:attach-descriptor (attach-descriptor) @ spark-core_2.10 ---
[INFO] 
[INFO] --- maven-source-plugin:2.2.1:jar-no-fork (create-source-jar) @ spark-core_2.10 ---
[INFO] Building jar: /mnt/devel/rahuls/code/apache-spark/core/target/spark-core_2.10-1.1.0-SNAPSHOT-sources.jar
[INFO] 
[INFO] --- scalastyle-maven-plugin:0.4.0:check (default) @ spark-core_2.10 ---
Saving to outputFile=/mnt/devel/rahuls/code/apache-spark/core/scalastyle-output.xml
Processed 361 file(s)
Found 0 errors
Found 0 warnings
Found 0 infos
Finished in 8234 ms

Another way to run these checks is through `mvn scalastyle:check` but that requires the inter-module dependencies are satisfied via M2 repo.