Working on undo

Does not work yet, the score undos but not the grid
Added undo

The undo button now works correctly. However, it is slightly
ineffiecent because it still stores the move history even when there
are no undos left. I may keep it this way though because I plan on
adding the ability to purchase more undos.
Moves while animation is playing are ignored

If you swiped while the tiles were already moving the animations would
get messed up. Now it ignores new moves to prevent conflicts.
Merge pull request #3 from tytan34/beta

Beta