First version uploaded

Uploaded the first version of my checkers. It's playable, and the game enforces the rules.

Known problems: 

* Sometimes the selected square does not register, causing the stone to stay at the starting square.

Plans for the future:

* Fix ugly scaling/aliasing issues.

* Add movement instead of instant snapping to the correct square.

* Add stone capture animation.

* Add captured stones counter (?).

* Add current player indicator.

* Enable undoing moves.

* Add stone setup animation.

* Add endgame screen.

* Add AI with difficulty slider.