Refactor code to set puzzleboard.

Removed duplication of code that initially set puzzleboard and that reset the puzzleboard. Deleted clearlastgame function as it was no longer necessary.  When the shuffle button is clicked, the 'shuffled' class is now added to the puzzleboard instead of the shuffle button.
Refactor JS code.

Make emptyTile a global variable.  Create setShuffledPuzzleBoard function to hold code related the shuffled state.  Delete a couple of unnecessary variables.