15 is a game where the goal is to force your opponent to make the number go over 15. On one's turn, one can add 1, 2, or 3 to the current number.
As usual, all of @coocoo333's ununderstandable code was removed and reworked. You can now play by yourself against a simple program. (It is possible to beat the program, but I won't give away any strategies.) Also, it seems I spent 9 months not sharing this project when it was pretty much done, I don't know why.
