Bugfix + checkstyle + minor API breaks

• Minor API breaks coming from introduction of final methods that were previously not final. Should be not dramatic.
• Bugfix for isValid() which returned wrong results in some situations (see #2 for details ).
• Checkstyle adjustments resulting in cleaner code.

Some Sudoku examples

2000 Sudokus is an example of the LaTeX output produced by the CLI client.