implemented Checkers::Judge#valid_moves_from for white pawn
- removes moves to positions that are out of bounds or engaged
- tested
Checkers::Board::Position#new no longer validates coordinates
- added valid? predicate, validate method (raises errors) and new! validating constructor
added Checkers::Board#free?(position)
implemented Checkers::Judge#valid_move? by means of valid_moves_from
- tested
prepaired test for moves of black pawn
Judge#valid_moves_from now returns correct moves for black pawn
- added Board::Position#forward, #left and #right, these take color
- tested