corrected the method isMoveValid for Pawn to account for the fact that Black pieces move "backwards"; added private methods that clean up the code in determining how a pawn moves based on its color; closes #8
updated Bishop's isMoveValid method
removed isValid for location, delegating that to "higher powers" to check validity; added getValidLocationsTo for Bishop; added width and height as dimensions to Board
added an isLocationValid method to the Board class
added getValidLocations to the Bishop class
moved setLocation to the Piece class
made the getValidLocationsTo method for Rook class