Enable Maven-checkstyle-plugin

I noticed that I don't get any build errors or warnings with unused imports.

I thought there is a checkstyle plugin running as part of Apache reports.

But there isn't. 

There is a plugin here: http://maven.apache.org/plugins/maven-checkstyle-plugin/index.html

I think it reveals a lot of useful information. Doesn't mean we fix all of it. But its very helpful.

It may also generate a nice report:

http://maven.apache.org/plugins-archives/maven-project-info-reports-plugin-LATEST/checkstyle.html

Which can be useful once more people work on code, so you don't need to review syntax and code smells in every PR manually.