Initial version of libgames-scores

Fake pull request for initial review of libgames-scores.
Traditionally, a comparison function is not required to return -1, 0, or 1, and would just be implemented more simply by doing `return b.score - a.score`.  This is what the next programmer down the line is going to expect to see. Does the documentation say it has to be -1, 0, or 1?  If not, try changing these both to subtraction and see if it still works; I bet it will.