Useless git comment!

c2048.[ch]:
- renamed MAX_BOARD to BOARD_MAX (more consistent)
- renamed any reference from cell to tile to be more
  consistant to the game board.
- added some comments to _c2048_do_collapse _c2048_do_move.

c2048.c:
- renamed any reference from cell to tile to be more
  consistant to the game board theme.