Clean up build system

* Remove playgame.vala -- I think we can keep everything in ai.vala (at least for now)
* Remove playgame.h, which should not have been committed since it's generated by valac
* Make sure ai.vala is always compiled before ai.h is needed

I don't know how interested you are in Automake -- let me know if you want more explanation for what I'm doing here.