completed? method finished for sudoku class.  Will run through the board layout and return false if any cell value is 0
solve! method will now fill available cells until the board is completed.  It iterates through the board and for any cell with a value of 0 it will find potential values based on that cell's row/col/box. If only one potential value, it assigns that value to the cell and updates rows/cols/boxes.
Added original_board instance variable to Sudoku class.  This will be used later in the display function. Also removed Unnecessary @ symbols within different methods
Original board input and Solved board layout are printed upon completion.