Fixed a bug, in a case when there are two cells left, and one of them is
a mine, you would win even if you clicked on a mine. Simply swapped the
mine checking code in front of cellCleared function.

Also added a convenience feature, which guarantees that the first click
on the board never contains a mine (it was annoying).

Also fixed another mild annoyance: when reseting the game by pressing
the space bar, the cell under the cursor would not be highlighted and so
could not be pressed until the cursor leaves and re-enters that cell.
Now you don't have to do that.