Adds perform_slide, [], and []= methods to Board class.

perform_slide moves a piece from one location to another on the board. The new
[] and []= methods allow for access to the array underlying the Board using
a two-dimensional array as an argument. board_instance[[0, 0]] now returns
board_instance.board[0][0], for example.
Rewrites movement methods to account for current location

slide_moves and jump_moves now return an array of possible destinations
after taking into account the Piece's starting position. Previously, they had
only returned lists of possible offsets after a move.
Adds initialize method to Checkers

Also adds EOF to end of all files
Adds error handling to perform_slide