Add baffling-birthdays exercise

[kahgoh]

The baffling-birthdays exercise was recently added to the problem specifications. This task is to add the practice exercise to the Java track.

[habbie321]

Could I claim this one?

[kahgoh]

Yes, I've assigned this one to you @habbie321. If this is your first time adding a practice exercise, check out the CONTRIBUTING.md and Exercism's Practice Exercises. Let me know if you need further help or have any further questions!