Fix checkstyle and replace all main() on tests

[vbeskrovnov]

Hello! What are you thinking about integrating this project with maven, adding checkstyle rules and replace all main() methods on unit tests? Because now it's very difficult to read and understand code because of bad checkstyle.

[xaca]

@vbeskrovnov Please hel us with that.

[fndejong]

As a start, I created a PR that introduces Maven and JUnit into the project. To simplify things a bit for beginners, tests can write information to System.out which will be displayed in the console.

I did not include CheckStyle.

See PR-107: https://github.com/xaca/java_fundamentals/pull/107