Add mandatory checkstyle checks to the maven build.

This MR adds checkstyle as a mandatory build step to the maven builds.
I went with the default Google Style for now, because so far there isn't anything defined afaik and it is already available by default in most IDEs.

The checkstyle plugin will now always fail the build if it detects a violation. Similar to what rubocop does for ruby.

This is based on !2 (merged) and closes #9

Still a WIP, because I still need to adjust the actual code.
I'm at a conference at the moment, so I'll do that tomorrow.

edit: done. removing the WIP.