* isbn: implement exercise in Java track

* isbn-verifier: Update reference solution.

* isbn-verifier: Add starter implementation.

* Add @FridaTveit 's improvements

* Add some more improvements

* Added palindrome-products exercise to the Java track

* Added @Ignore annotations to tests

* Added binary search tree exercise to the Java track

* Removed pythagorial triplet from config.json

* Removed binary search from config.json