Added intFile() and charFile() methods to BoardImpl for easy int-char
conversions in the case of files.
Implemented private Board constructor to create empty board.
Implemented BoardImpl.update()
Added BoardFactoryImpl implementation of BoardFactory. Implemented
emptyBoard() and adding pawns in newBoard().
Added SquareImpl implementation of Square. Included protected
constructor for use by model classes.
Added BoardUtils to the utils package. Currently only contains methods
for transforming between the char (a-h) and int (1-8) representations of
files.
Updated inheritance structure by making Piece into an interface, because
it didn't contain any useful reusable code and I didn't forsee it
containing any in the future.
Cleaned up method names in a few places. See below for details.