Changing some variables to protected for extension.
More ACM changes in GridBoard.

Also adding the class responsible for all these changes, BlankBoard.
Modifications to BlankBoard and unit-test spec.
Adding implementation for addPiece.

However, tests are still failing :C.
Add piece for black pieces.
What's wrong is that Pawns are not even represented in ARRANGEMENT arrays!
Tests are now passing.

We relaxed the ACM on the wrong field in GridBoard. Read the docstrings you spend
so much time to write!