Fourth version uploaded

Uploaded the fourth version of my checkers.

Added:

* Undo/Redo buttons (not functional yet), that double as a current player indicator.

* The most basic AI (does first possible move).

Plans for the future:

* Wrong move animation (highlight mandatory capture).

* Fix drag'n'drop.

* Add touchscreen support.

* Fix ugly scaling/aliasing issues.

* Enable undoing moves.

* Add AI with difficulty slider.