Updates road checks to include the check that the road is built off
another road or a settlement. Added buying road functions in Player and
GameBoard. Changed the corners variable in GameBoard to use CornerPieces
instead of GamePieces. Renamed "setters" in Player class, they are now
called adders like their functionality.
Merge pull request #5 from Databean/road_implementation

Updates road checks to include the check that the road is built off