Introduce maven-checkstyle-plugin to validate no star imports

[sghill]

It'd be great to have the build validate style rules for newer contributors to the project. I added the AvoidStarImport rule through the maven-checkstyle-plugin based on feedback from #516.

Here's how it fails when there is a star import:

[INFO] --- maven-checkstyle-plugin:3.0.0:check (validate) @ slack ---
[INFO] Starting audit...
[ERROR] <repo>/slack-plugin/src/main/java/jenkins/plugins/slack/ActiveNotifier.java:6: Using the '.*' form of import should be avoided - hudson.model.*. [AvoidStarImport]
Audit done.
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  4.656 s
[INFO] Finished at: 2019-02-15T14:09:19-08:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-checkstyle-plugin:3.0.0:check (validate) on project slack: Failed during checkstyle execution: There is 1 error reported by Checkstyle 6.18 with checkstyle.xml ruleset. -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException

[timja]

Thanks, I'll build on this 👍