Big Numbers representation

Your task is to:

1. finish the implementation of BigInt class. Provide methods for addition, multiplication and string representation of numbers as well as parsing big numbers from string (you can assume that the input String number is valid number representation that may start with - and then contains only digits).
2. provide some more tests to prove that your implementation works - you have to write 2 of them by yourself. The tests can be found in BigIntTest and you can run them locally by running

   ./gradlew test

   from the repository directory in your shell.
3. Open pull request to my repository and wait for review (it'll be public).