Use CheckStyle to verify proper formatting of source code

When changing the headers in the codebase to comply with ASL 2.0, I noticed there were at least 20 variations of the old header. And I routinely have to correct JavaDoc problems and compiler warnings in the source code.

To maintain consistency of the code, we should start using CheckStyle with a custom set of rules that works for our project. The checkstyle Maven plugin runs after the tests, and for most projects it runs in just a few seconds; it should have minimal impact on development workflow.