Implemented bth_is_valid_move, added tests;

I've done some work, mostly in reversi.c and testall.c. Basically, I've
implemented (and wrote tests for) bth_is_valid_move(), which will
probably be the backbone of the entire program.

I'm thinking about how to implement bth_get_valid_moves() in a better way
than just calling bth_is_valid_move() for every cell, but I'll probably
leave that to after the thing is functional.

There are still a few minor, boilerplate functions I have implemented, but
have not yet written tests for.