Replace getBoard() calls with getPiece().

I've been accessing pieces with getBoard().get(x).get(y). This violates
encapsulation, so I'm now using my new getPiece() methods to access the
locations on the ChessBoard. I may in the future get rid of the
getBoard() method, this is the first step in doing so.
Remove getBoard() from ChessBoard.

getBoard() violates encapsulation, so I've removed it. I also removed
the tests that used getBoard().